diff --git a/python_pkg/screen_locker/_constants.py b/python_pkg/screen_locker/_constants.py
index acc0e58..72ea665 100644
--- a/python_pkg/screen_locker/_constants.py
+++ b/python_pkg/screen_locker/_constants.py
@@ -11,7 +11,11 @@ ADB_TIMEOUT = 15
 STRONGLIFTS_DB_REMOTE = (
     "/data/data/com.stronglifts.app/databases/StrongLifts-Database-3"
 )
+MIN_WORKOUT_DURATION_MINUTES = 50
+MAX_CLOCK_SKEW_SECONDS = 300  # 5 minutes max time skew from NTP
 SHUTDOWN_CONFIG_FILE = Path("/etc/shutdown-schedule.conf")
+# HMAC key for signing workout log entries (root-owned, 0600)
+HMAC_KEY_FILE = Path("/etc/workout-locker/hmac.key")
 # Helper script path (relative to this file)
 ADJUST_SHUTDOWN_SCRIPT = Path(__file__).resolve().parent / "adjust_shutdown_schedule.sh"
 # State file to track sick day usage and original config values
diff --git a/python_pkg/screen_locker/_log_integrity.py b/python_pkg/screen_locker/_log_integrity.py
new file mode 100644
index 0000000..a34c6a2
--- /dev/null
+++ b/python_pkg/screen_locker/_log_integrity.py
@@ -0,0 +1,78 @@
+"""HMAC-based integrity checking for workout log entries."""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import json
+import logging
+import secrets
+
+from python_pkg.screen_locker._constants import HMAC_KEY_FILE
+
+_logger = logging.getLogger(__name__)
+
+
+def _load_hmac_key() -> bytes | None:
+    """Load HMAC key from the root-owned key file.
+
+    Returns the key bytes, or None if the file cannot be read.
+    """
+    try:
+        return HMAC_KEY_FILE.read_bytes().strip()
+    except OSError:
+        _logger.warning("Cannot read HMAC key from %s", HMAC_KEY_FILE)
+        return None
+
+
+def _generate_hmac_key() -> bytes | None:
+    """Generate a new HMAC key and write it to the key file.
+
+    The key file must be writable (requires root or setup script).
+    Returns the new key bytes, or None on failure.
+    """
+    key = secrets.token_bytes(32)
+    try:
+        HMAC_KEY_FILE.parent.mkdir(parents=True, exist_ok=True)
+        HMAC_KEY_FILE.write_bytes(key)
+    except OSError:
+        _logger.warning("Cannot write HMAC key to %s", HMAC_KEY_FILE)
+        return None
+    return key
+
+
+def compute_entry_hmac(entry_data: dict[str, object]) -> str | None:
+    """Compute HMAC-SHA256 for a workout log entry.
+
+    Args:
+        entry_data: The log entry dict (without the 'hmac' field).
+
+    Returns:
+        Hex-encoded HMAC string, or None if the key is unavailable.
+    """
+    key = _load_hmac_key()
+    if key is None:
+        return None
+    payload = json.dumps(entry_data, sort_keys=True, separators=(",", ":"))
+    return hmac.new(key, payload.encode(), hashlib.sha256).hexdigest()
+
+
+def verify_entry_hmac(entry: dict[str, object]) -> bool:
+    """Verify HMAC signature of a workout log entry.
+
+    Args:
+        entry: The full log entry dict including the 'hmac' field.
+
+    Returns:
+        True if the HMAC is valid, False if invalid or key unavailable.
+    """
+    stored_hmac = entry.get("hmac")
+    if not isinstance(stored_hmac, str):
+        return False
+    key = _load_hmac_key()
+    if key is None:
+        return False
+    entry_without_hmac = {k: v for k, v in entry.items() if k != "hmac"}
+    payload = json.dumps(entry_without_hmac, sort_keys=True, separators=(",", ":"))
+    expected = hmac.new(key, payload.encode(), hashlib.sha256).hexdigest()
+    return hmac.compare_digest(stored_hmac, expected)
diff --git a/python_pkg/screen_locker/_phone_verification.py b/python_pkg/screen_locker/_phone_verification.py
index 5d158ef..03fa8f7 100644
--- a/python_pkg/screen_locker/_phone_verification.py
+++ b/python_pkg/screen_locker/_phone_verification.py
@@ -11,8 +11,14 @@ import socket
 import sqlite3
 import subprocess
 import tempfile
+import time
 
-from python_pkg.screen_locker._constants import ADB_TIMEOUT, STRONGLIFTS_DB_REMOTE
+from python_pkg.screen_locker._constants import (
+    ADB_TIMEOUT,
+    MIN_WORKOUT_DURATION_MINUTES,
+    STRONGLIFTS_DB_REMOTE,
+)
+from python_pkg.screen_locker._time_check import check_clock_skew
 
 _logger = logging.getLogger(__name__)
 
@@ -179,25 +185,160 @@ class PhoneVerificationMixin:
             _logger.warning("Failed to query StrongLifts database")
             return 0
 
+    def _get_today_workout_duration_minutes(self, db_path: Path) -> float:
+        """Get the total duration in minutes of today's workouts.
+
+        Args:
+            db_path: Path to the locally-pulled StrongLifts database.
+
+        Returns:
+            Total duration in minutes of all workouts started today.
+            Returns 0.0 on any error or if no workouts found.
+        """
+        try:
+            conn = sqlite3.connect(str(db_path))
+            try:
+                cursor = conn.execute(
+                    "SELECT SUM((finish - start) / 1000.0 / 60.0) "
+                    "FROM workouts "
+                    "WHERE date(start / 1000, 'unixepoch', 'localtime') "
+                    "= date('now', 'localtime') "
+                    "AND finish > start",
+                )
+                row = cursor.fetchone()
+                return float(row[0]) if row and row[0] is not None else 0.0
+            finally:
+                conn.close()
+        except (sqlite3.Error, ValueError, TypeError):
+            _logger.warning("Failed to query workout duration")
+            return 0.0
+
+    def _get_today_exercise_count(self, db_path: Path) -> int:
+        """Count distinct exercises in today's workouts.
+
+        Uses the StrongLifts 'exercises' table joined with 'workouts' to
+        verify that actual exercises were logged, not just empty sessions.
+
+        Args:
+            db_path: Path to the locally-pulled StrongLifts database.
+
+        Returns:
+            Number of distinct exercises in today's workouts.
+            Returns 0 on any error.
+        """
+        try:
+            conn = sqlite3.connect(str(db_path))
+            try:
+                cursor = conn.execute(
+                    "SELECT COUNT(DISTINCT e.exercise) "
+                    "FROM exercises e "
+                    "JOIN workouts w ON e.workout = w.id "
+                    "WHERE date(w.start / 1000, 'unixepoch', 'localtime') "
+                    "= date('now', 'localtime')",
+                )
+                row = cursor.fetchone()
+                return int(row[0]) if row else 0
+            finally:
+                conn.close()
+        except (sqlite3.Error, ValueError, TypeError):
+            _logger.warning("Failed to query exercise count")
+            return 0
+
+    def _is_workout_finish_recent(self, db_path: Path) -> bool:
+        """Check if the latest workout's finish time is recent.
+
+        A fresh workout should have finished within the last few hours.
+        This prevents using an old pre-prepared database dump.
+
+        Args:
+            db_path: Path to the locally-pulled StrongLifts database.
+
+        Returns:
+            True if the latest finish time is within 4 hours of now.
+        """
+        max_age_seconds = 4 * 3600
+        try:
+            conn = sqlite3.connect(str(db_path))
+            try:
+                cursor = conn.execute(
+                    "SELECT MAX(finish) FROM workouts "
+                    "WHERE date(start / 1000, 'unixepoch', 'localtime') "
+                    "= date('now', 'localtime') "
+                    "AND finish > start",
+                )
+                row = cursor.fetchone()
+                if not row or row[0] is None:
+                    return False
+                finish_epoch = int(row[0]) / 1000.0
+                return (time.time() - finish_epoch) < max_age_seconds
+            finally:
+                conn.close()
+        except (sqlite3.Error, ValueError, TypeError):
+            _logger.warning("Failed to query workout finish time")
+            return False
+
+    def _validate_workout_db(
+        self,
+        local_db: Path,
+    ) -> tuple[str, str] | None:
+        """Validate workout database has a recent, real workout.
+
+        Returns:
+            A (status, message) tuple if validation fails, or None if OK.
+        """
+        count = self._count_today_workouts(local_db)
+        if count <= 0:
+            return "not_verified", "No workout found on phone today"
+        if not self._is_workout_finish_recent(local_db):
+            return (
+                "stale",
+                "Workout finish time is too old. Did you actually work out today?",
+            )
+        exercise_count = self._get_today_exercise_count(local_db)
+        if exercise_count < 1:
+            return (
+                "no_exercises",
+                "No exercises found in today's workout. "
+                "Log actual exercises in StrongLifts!",
+            )
+        return None
+
     def _verify_phone_workout(self) -> tuple[str, str]:
         """Verify workout was recorded in StrongLifts on the phone.
 
         Returns:
             Tuple of (status, message) where status is one of:
-            - "verified": Workout confirmed on phone.
+            - "verified": Workout confirmed and >= minimum duration.
+            - "too_short": Workout found but shorter than minimum.
             - "not_verified": Phone connected but no workout found.
             - "no_phone": No phone connected via ADB.
             - "error": Could not access StrongLifts database.
+            - "stale": Workout finish time is not recent.
+            - "no_exercises": Workout has no logged exercises.
+            - "clock_tampered": System clock skew exceeds threshold.
         """
+        clock_ok, clock_msg = check_clock_skew()
+        if not clock_ok:
+            return "clock_tampered", clock_msg
         if not self._is_phone_connected():
             return "no_phone", "No phone connected via ADB"
         local_db = self._pull_stronglifts_db()
         if local_db is None:
             return "error", "StrongLifts database not found on phone"
-        count = self._count_today_workouts(local_db)
-        if count > 0:
+        db_error = self._validate_workout_db(local_db)
+        if db_error is not None:
+            return db_error
+        duration = self._get_today_workout_duration_minutes(local_db)
+        if duration < MIN_WORKOUT_DURATION_MINUTES:
             return (
-                "verified",
-                f"Workout verified! ({count} session(s) found on phone)",
+                "too_short",
+                f"Workout too short! {duration:.0f} min logged, "
+                f"need at least {MIN_WORKOUT_DURATION_MINUTES} min.",
             )
-        return "not_verified", "No workout found on phone today"
+        exercise_count = self._get_today_exercise_count(local_db)
+        return (
+            "verified",
+            f"Workout verified! ({self._count_today_workouts(local_db)}"
+            f" session(s), {duration:.0f} min, "
+            f"{exercise_count} exercise(s))",
+        )
diff --git a/python_pkg/screen_locker/_time_check.py b/python_pkg/screen_locker/_time_check.py
new file mode 100644
index 0000000..f1d0073
--- /dev/null
+++ b/python_pkg/screen_locker/_time_check.py
@@ -0,0 +1,79 @@
+"""System clock skew detection via NTP."""
+
+from __future__ import annotations
+
+import logging
+import socket
+import struct
+import time
+
+from python_pkg.screen_locker._constants import MAX_CLOCK_SKEW_SECONDS
+
+_logger = logging.getLogger(__name__)
+
+_NTP_EPOCH_OFFSET = 2208988800  # Seconds between 1900-01-01 and 1970-01-01
+_NTP_PORT = 123
+_NTP_TIMEOUT = 5
+_NTP_MIN_PACKET_SIZE = 48
+
+
+def _query_ntp_offset(server: str = "pool.ntp.org") -> float | None:
+    """Query an NTP server and return the clock offset in seconds.
+
+    Uses a minimal SNTP (RFC 4330) client-mode request.
+
+    Returns:
+        Offset in seconds (positive = local clock is ahead), or None on error.
+    """
+    # NTP v3, mode 3 (client), transmit timestamp at bytes 40-47
+    packet = b"\x1b" + b"\0" * 47
+    try:
+        with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as sock:
+            sock.settimeout(_NTP_TIMEOUT)
+            t1 = time.time()
+            sock.sendto(packet, (server, _NTP_PORT))
+            data, _ = sock.recvfrom(1024)
+            t4 = time.time()
+    except OSError as exc:
+        _logger.info("NTP query to %s failed: %s", server, exc)
+        return None
+
+    if len(data) < _NTP_MIN_PACKET_SIZE:
+        return None
+
+    # Transmit timestamp from server (bytes 40-47)
+    tx_seconds = struct.unpack("!I", data[40:44])[0] - _NTP_EPOCH_OFFSET
+    tx_fraction = struct.unpack("!I", data[44:48])[0] / (2**32)
+    server_time = tx_seconds + tx_fraction
+
+    # Simplified offset: server_time should be close to (t1 + t4) / 2
+    local_mid = (t1 + t4) / 2
+    return server_time - local_mid
+
+
+def check_clock_skew() -> tuple[bool, str]:
+    """Check if system clock is within acceptable skew of NTP time.
+
+    Returns:
+        Tuple of (ok, message).
+        ok is True if clock is within MAX_CLOCK_SKEW_SECONDS or NTP is unreachable.
+        When NTP is unreachable, we allow through (fail-open for network issues).
+    """
+    offset = _query_ntp_offset()
+    if offset is None:
+        _logger.info("NTP unreachable — allowing through")
+        return True, "NTP check skipped (server unreachable)"
+
+    abs_offset = abs(offset)
+    if abs_offset > MAX_CLOCK_SKEW_SECONDS:
+        direction = "ahead" if offset < 0 else "behind"
+        _logger.warning(
+            "Clock skew detected: %.0f seconds %s",
+            abs_offset,
+            direction,
+        )
+        return False, (
+            f"System clock is {abs_offset:.0f}s {direction} of NTP time. "
+            f"Max allowed skew: {MAX_CLOCK_SKEW_SECONDS}s."
+        )
+    return True, f"Clock OK (offset: {offset:+.1f}s)"
diff --git a/python_pkg/screen_locker/_ui_flows.py b/python_pkg/screen_locker/_ui_flows.py
index 96e93d6..489e44b 100644
--- a/python_pkg/screen_locker/_ui_flows.py
+++ b/python_pkg/screen_locker/_ui_flows.py
@@ -70,6 +70,25 @@ class UIFlowsMixin:
             self._text("Unlocking...", font_size=18, color="#888888")
             unlock_delay = 1500 if self.demo_mode else 2000
             self.root.after(unlock_delay, self.unlock_screen)
+        elif status == "too_short":
+            self._show_retry_and_sick(
+                f"\u274c {message}\n\n"
+                "Your workout was too short!\n"
+                "Actually do the full workout, don't just\n"
+                "spam through the exercises.",
+            )
+        elif status in ("stale", "no_exercises"):
+            self._show_retry_and_sick(
+                f"\u274c {message}\n\n"
+                "The workout data looks suspicious.\n"
+                "Make sure you did a real workout today.",
+            )
+        elif status == "clock_tampered":
+            self._show_retry_and_sick(
+                f"\u274c {message}\n\n"
+                "System clock appears to be manipulated.\n"
+                "Fix your system time and try again.",
+            )
         elif status == "not_verified":
             self._show_retry_and_sick(
                 f"\u274c {message}\n\n"
diff --git a/python_pkg/screen_locker/install_systemd.sh b/python_pkg/screen_locker/install_systemd.sh
index 703e3cc..3fd3f83 100755
--- a/python_pkg/screen_locker/install_systemd.sh
+++ b/python_pkg/screen_locker/install_systemd.sh
@@ -66,6 +66,9 @@ if [ -f "$I3_CONFIG" ] && grep -q "exec.*screen_lock.py" "$I3_CONFIG"; then
 	echo "✓ i3 autostart: INSTALLED"
 else
 	echo "  i3 autostart: not installed"
+	echo ""
+	echo "To add i3 startup hook (recommended), add this line to $I3_CONFIG:"
+	echo "  exec --no-startup-id /usr/bin/python3 -m python_pkg.screen_locker.screen_lock --production"
 fi
 
 # Immediately check if today's workout is done; block if not
diff --git a/python_pkg/screen_locker/screen_lock.py b/python_pkg/screen_locker/screen_lock.py
index 9bbe436..43f5a95 100755
--- a/python_pkg/screen_locker/screen_lock.py
+++ b/python_pkg/screen_locker/screen_lock.py
@@ -15,27 +15,37 @@ import sys
 import tkinter as tk
 from typing import TYPE_CHECKING
 
-if TYPE_CHECKING:
-    from collections.abc import Callable
-    from concurrent.futures import Future
-
 from python_pkg.screen_locker._constants import (
+    HMAC_KEY_FILE,
+    MAX_CLOCK_SKEW_SECONDS,
+    MIN_WORKOUT_DURATION_MINUTES,
     PHONE_PENALTY_DELAY_DEMO,
     PHONE_PENALTY_DELAY_PRODUCTION,
     SICK_LOCKOUT_SECONDS,
     STRONGLIFTS_DB_REMOTE,
 )
+from python_pkg.screen_locker._log_integrity import (
+    compute_entry_hmac,
+    verify_entry_hmac,
+)
+from python_pkg.screen_locker._phone_verification import PhoneVerificationMixin
+from python_pkg.screen_locker._shutdown import ShutdownMixin
+from python_pkg.screen_locker._ui_flows import UIFlowsMixin
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+    from concurrent.futures import Future
 
 __all__ = [
+    "HMAC_KEY_FILE",
+    "MAX_CLOCK_SKEW_SECONDS",
+    "MIN_WORKOUT_DURATION_MINUTES",
     "PHONE_PENALTY_DELAY_DEMO",
     "PHONE_PENALTY_DELAY_PRODUCTION",
     "SICK_LOCKOUT_SECONDS",
     "STRONGLIFTS_DB_REMOTE",
     "ScreenLocker",
 ]
-from python_pkg.screen_locker._phone_verification import PhoneVerificationMixin
-from python_pkg.screen_locker._shutdown import ShutdownMixin
-from python_pkg.screen_locker._ui_flows import UIFlowsMixin
 
 _logger = logging.getLogger(__name__)
 
@@ -252,7 +262,7 @@ class ScreenLocker(
         self.root.after(1500, self.close)
 
     def has_logged_today(self) -> bool:
-        """Check if workout has been logged today."""
+        """Check if workout has been logged today with valid HMAC."""
         if not self.log_file.exists():
             return False
 
@@ -263,7 +273,13 @@ class ScreenLocker(
             return False
         else:
             today = datetime.now(tz=timezone.utc).strftime("%Y-%m-%d")
-            return today in logs
+            entry = logs.get(today)
+            if entry is None:
+                return False
+            if not verify_entry_hmac(entry):
+                _logger.warning("HMAC verification failed for today's log entry")
+                return False
+            return True
 
     def _load_existing_logs(self) -> dict:
         """Load existing workout logs from file."""
@@ -276,13 +292,19 @@ class ScreenLocker(
             return {}
 
     def save_workout_log(self) -> None:
-        """Save workout data to log file."""
+        """Save workout data to log file with HMAC signature."""
         logs = self._load_existing_logs()
         today = datetime.now(tz=timezone.utc).strftime("%Y-%m-%d")
-        logs[today] = {
+        entry: dict[str, object] = {
             "timestamp": datetime.now(tz=timezone.utc).isoformat(),
             "workout_data": self.workout_data,
         }
+        signature = compute_entry_hmac(entry)
+        if signature is not None:
+            entry["hmac"] = signature
+        else:
+            _logger.warning("HMAC key unavailable — saving unsigned entry")
+        logs[today] = entry
         try:
             with self.log_file.open("w") as f:
                 json.dump(logs, f, indent=2)
diff --git a/python_pkg/screen_locker/tests/test_adb_and_phone.py b/python_pkg/screen_locker/tests/test_adb_and_phone.py
index d0ec67c..7b79b5d 100644
--- a/python_pkg/screen_locker/tests/test_adb_and_phone.py
+++ b/python_pkg/screen_locker/tests/test_adb_and_phone.py
@@ -8,6 +8,8 @@ import time
 from typing import TYPE_CHECKING
 from unittest.mock import MagicMock, patch
 
+import pytest
+
 from python_pkg.screen_locker.screen_lock import STRONGLIFTS_DB_REMOTE
 from python_pkg.screen_locker.tests.conftest import create_locker
 
@@ -473,3 +475,329 @@ class TestCountTodayWorkouts:
         conn.close()
 
         assert locker._count_today_workouts(db_file) == 2
+
+
+class TestGetTodayWorkoutDurationMinutes:
+    """Tests for _get_today_workout_duration_minutes method."""
+
+    def test_returns_duration_for_today_workout(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns correct duration for a 60-minute workout."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        now_ms = int(time.time() * 1000)
+        duration_ms = 60 * 60 * 1000  # 60 minutes
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", now_ms, now_ms + duration_ms),
+        )
+        conn.commit()
+        conn.close()
+
+        result = locker._get_today_workout_duration_minutes(db_file)
+        assert result == pytest.approx(60.0, abs=1.0)
+
+    def test_returns_zero_for_no_workouts(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns 0.0 when no workouts today."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        yesterday_ms = int((time.time() - 200000) * 1000)
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", yesterday_ms, yesterday_ms + 3600000),
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._get_today_workout_duration_minutes(db_file) == 0.0
+
+    def test_sums_multiple_workouts(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test sums durations of multiple workouts today."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        now_ms = int(time.time() * 1000)
+        # 30 min + 25 min = 55 min total
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", now_ms, now_ms + 30 * 60 * 1000),
+        )
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w2", now_ms + 31 * 60 * 1000, now_ms + 56 * 60 * 1000),
+        )
+        conn.commit()
+        conn.close()
+
+        result = locker._get_today_workout_duration_minutes(db_file)
+        assert result == pytest.approx(55.0, abs=1.0)
+
+    def test_ignores_invalid_finish(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test ignores workouts where finish <= start."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        now_ms = int(time.time() * 1000)
+        # finish == start (zero duration - should be excluded by WHERE)
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", now_ms, now_ms),
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._get_today_workout_duration_minutes(db_file) == 0.0
+
+    def test_invalid_db_returns_zero(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns 0.0 for invalid database file."""
+        locker = create_locker(mock_tk, tmp_path)
+        bad_file = tmp_path / "not_a_db.db"
+        bad_file.write_text("not a database")
+
+        assert locker._get_today_workout_duration_minutes(bad_file) == 0.0
+
+    def test_missing_table_returns_zero(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns 0.0 when workouts table doesn't exist."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "empty.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute("CREATE TABLE other (id TEXT)")
+        conn.commit()
+        conn.close()
+
+        assert locker._get_today_workout_duration_minutes(db_file) == 0.0
+
+
+class TestGetTodayExerciseCount:
+    """Tests for _get_today_exercise_count method."""
+
+    def test_counts_exercises(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test counts distinct exercises in today's workouts."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        conn.execute(
+            "CREATE TABLE exercises (id TEXT, workout TEXT, exercise TEXT)",
+        )
+        now_ms = int(time.time() * 1000)
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", now_ms, now_ms + 3600000),
+        )
+        conn.execute(
+            "INSERT INTO exercises VALUES (?, ?, ?)",
+            ("e1", "w1", "squat"),
+        )
+        conn.execute(
+            "INSERT INTO exercises VALUES (?, ?, ?)",
+            ("e2", "w1", "bench_press"),
+        )
+        conn.execute(
+            "INSERT INTO exercises VALUES (?, ?, ?)",
+            ("e3", "w1", "squat"),
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._get_today_exercise_count(db_file) == 2
+
+    def test_no_exercises_returns_zero(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns 0 when no exercises exist."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        conn.execute(
+            "CREATE TABLE exercises (id TEXT, workout TEXT, exercise TEXT)",
+        )
+        now_ms = int(time.time() * 1000)
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", now_ms, now_ms + 3600000),
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._get_today_exercise_count(db_file) == 0
+
+    def test_invalid_db_returns_zero(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns 0 for invalid database file."""
+        locker = create_locker(mock_tk, tmp_path)
+        bad_file = tmp_path / "bad.db"
+        bad_file.write_text("not a db")
+
+        assert locker._get_today_exercise_count(bad_file) == 0
+
+    def test_missing_table_returns_zero_exercises(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns 0 when exercises table doesn't exist."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "empty.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._get_today_exercise_count(db_file) == 0
+
+
+class TestIsWorkoutFinishRecent:
+    """Tests for _is_workout_finish_recent method."""
+
+    def test_recent_workout_returns_true(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns True for workout that finished recently."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        now_ms = int(time.time() * 1000)
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", now_ms - 3600000, now_ms),
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._is_workout_finish_recent(db_file) is True
+
+    def test_old_workout_returns_false(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns False for workout that finished >4 hours ago."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        # Finished 5 hours ago (but still "today" in local time)
+        now_ms = int(time.time() * 1000)
+        old_finish = now_ms - 5 * 3600 * 1000
+        conn.execute(
+            "INSERT INTO workouts VALUES (?, ?, ?)",
+            ("w1", old_finish - 3600000, old_finish),
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._is_workout_finish_recent(db_file) is False
+
+    def test_no_workouts_returns_false(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns False when no workouts exist."""
+        locker = create_locker(mock_tk, tmp_path)
+        db_file = tmp_path / "sl_test.db"
+        conn = sqlite3.connect(str(db_file))
+        conn.execute(
+            "CREATE TABLE workouts "
+            "(id TEXT PRIMARY KEY, start INTEGER, finish INTEGER)",
+        )
+        conn.commit()
+        conn.close()
+
+        assert locker._is_workout_finish_recent(db_file) is False
+
+    def test_invalid_db_returns_false(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test returns False for invalid database file."""
+        locker = create_locker(mock_tk, tmp_path)
+        bad_file = tmp_path / "bad.db"
+        bad_file.write_text("not a db")
+
+        assert locker._is_workout_finish_recent(bad_file) is False
diff --git a/python_pkg/screen_locker/tests/test_init_and_log.py b/python_pkg/screen_locker/tests/test_init_and_log.py
index 58d02ac..a781e69 100644
--- a/python_pkg/screen_locker/tests/test_init_and_log.py
+++ b/python_pkg/screen_locker/tests/test_init_and_log.py
@@ -6,7 +6,7 @@ from datetime import datetime, timezone
 import json
 import tkinter as tk
 from typing import TYPE_CHECKING, Any
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 import pytest
 
@@ -101,14 +101,41 @@ class TestHasLoggedToday:
         mock_sys_exit: MagicMock,
         tmp_path: Path,
     ) -> None:
-        """Test when today's workout is logged."""
+        """Test when today's workout is logged with valid HMAC."""
         log_file = tmp_path / "workout_log.json"
         today = datetime.now(tz=timezone.utc).strftime("%Y-%m-%d")
-        log_file.write_text(json.dumps({today: {"workout": "data"}}))
+        log_file.write_text(
+            json.dumps({today: {"workout": "data", "hmac": "valid"}}),
+        )
 
         locker = create_locker(mock_tk, tmp_path)
         locker.log_file = log_file
-        assert locker.has_logged_today() is True
+        with patch(
+            "python_pkg.screen_locker.screen_lock.verify_entry_hmac",
+            return_value=True,
+        ):
+            assert locker.has_logged_today() is True
+
+    def test_today_logged_invalid_hmac(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test rejects entry when HMAC verification fails."""
+        log_file = tmp_path / "workout_log.json"
+        today = datetime.now(tz=timezone.utc).strftime("%Y-%m-%d")
+        log_file.write_text(
+            json.dumps({today: {"workout": "data", "hmac": "tampered"}}),
+        )
+
+        locker = create_locker(mock_tk, tmp_path)
+        locker.log_file = log_file
+        with patch(
+            "python_pkg.screen_locker.screen_lock.verify_entry_hmac",
+            return_value=False,
+        ):
+            assert locker.has_logged_today() is False
 
     def test_other_day_logged(
         self,
@@ -134,12 +161,16 @@ class TestSaveWorkoutLog:
         mock_sys_exit: MagicMock,
         tmp_path: Path,
     ) -> None:
-        """Test saving to a new log file."""
+        """Test saving to a new log file includes HMAC."""
         log_file = tmp_path / "workout_log.json"
         locker = create_locker(mock_tk, tmp_path)
         locker.log_file = log_file
         locker.workout_data = {"type": "running"}
-        locker.save_workout_log()
+        with patch(
+            "python_pkg.screen_locker.screen_lock.compute_entry_hmac",
+            return_value="abc123",
+        ):
+            locker.save_workout_log()
 
         assert log_file.exists()
         with log_file.open() as f:
@@ -147,6 +178,29 @@ class TestSaveWorkoutLog:
         today = datetime.now(tz=timezone.utc).strftime("%Y-%m-%d")
         assert today in data
         assert data[today]["workout_data"]["type"] == "running"
+        assert data[today]["hmac"] == "abc123"
+
+    def test_save_to_new_file_no_hmac_key(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test saving without HMAC key produces unsigned entry."""
+        log_file = tmp_path / "workout_log.json"
+        locker = create_locker(mock_tk, tmp_path)
+        locker.log_file = log_file
+        locker.workout_data = {"type": "running"}
+        with patch(
+            "python_pkg.screen_locker.screen_lock.compute_entry_hmac",
+            return_value=None,
+        ):
+            locker.save_workout_log()
+
+        with log_file.open() as f:
+            data: dict[str, Any] = json.load(f)
+        today = datetime.now(tz=timezone.utc).strftime("%Y-%m-%d")
+        assert "hmac" not in data[today]
 
     def test_save_to_existing_file(
         self,
@@ -161,7 +215,11 @@ class TestSaveWorkoutLog:
         locker = create_locker(mock_tk, tmp_path)
         locker.log_file = log_file
         locker.workout_data = {"type": "strength"}
-        locker.save_workout_log()
+        with patch(
+            "python_pkg.screen_locker.screen_lock.compute_entry_hmac",
+            return_value="sig",
+        ):
+            locker.save_workout_log()
 
         with log_file.open() as f:
             data: dict[str, Any] = json.load(f)
@@ -182,7 +240,11 @@ class TestSaveWorkoutLog:
         locker = create_locker(mock_tk, tmp_path)
         locker.log_file = log_file
         locker.workout_data = {"type": "running"}
-        locker.save_workout_log()
+        with patch(
+            "python_pkg.screen_locker.screen_lock.compute_entry_hmac",
+            return_value="sig",
+        ):
+            locker.save_workout_log()
 
         with log_file.open() as f:
             data: dict[str, Any] = json.load(f)
@@ -201,8 +263,12 @@ class TestSaveWorkoutLog:
         locker = create_locker(mock_tk, tmp_path)
         locker.log_file = log_file
         locker.workout_data = {"type": "running"}
-        # Should not raise, just log warning
-        locker.save_workout_log()
+        with patch(
+            "python_pkg.screen_locker.screen_lock.compute_entry_hmac",
+            return_value="sig",
+        ):
+            # Should not raise, just log warning
+            locker.save_workout_log()
 
 
 class TestRun:
diff --git a/python_pkg/screen_locker/tests/test_log_integrity.py b/python_pkg/screen_locker/tests/test_log_integrity.py
new file mode 100644
index 0000000..e3fb191
--- /dev/null
+++ b/python_pkg/screen_locker/tests/test_log_integrity.py
@@ -0,0 +1,152 @@
+"""Tests for _log_integrity HMAC signing and verification."""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import json
+from typing import TYPE_CHECKING
+from unittest.mock import patch
+
+from python_pkg.screen_locker._log_integrity import (
+    _generate_hmac_key,
+    _load_hmac_key,
+    compute_entry_hmac,
+    verify_entry_hmac,
+)
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+class TestLoadHmacKey:
+    """Tests for _load_hmac_key."""
+
+    def test_loads_key_from_file(self, tmp_path: Path) -> None:
+        """Test loading HMAC key from existing file."""
+        key_file = tmp_path / "hmac.key"
+        key_file.write_bytes(b"secret_key_bytes")
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            result = _load_hmac_key()
+        assert result == b"secret_key_bytes"
+
+    def test_returns_none_on_missing_file(self, tmp_path: Path) -> None:
+        """Test returns None when key file doesn't exist."""
+        key_file = tmp_path / "nonexistent.key"
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            result = _load_hmac_key()
+        assert result is None
+
+
+class TestGenerateHmacKey:
+    """Tests for _generate_hmac_key."""
+
+    def test_generates_and_writes_key(self, tmp_path: Path) -> None:
+        """Test key generation creates file with 32-byte key."""
+        key_file = tmp_path / "subdir" / "hmac.key"
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            result = _generate_hmac_key()
+        assert result is not None
+        assert len(result) == 32
+        assert key_file.read_bytes() == result
+
+    def test_returns_none_on_write_failure(self) -> None:
+        """Test returns None when file cannot be written."""
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+        ) as mock_path:
+            mock_path.parent.mkdir.side_effect = OSError("permission denied")
+            result = _generate_hmac_key()
+        assert result is None
+
+
+class TestComputeEntryHmac:
+    """Tests for compute_entry_hmac."""
+
+    def test_computes_hmac_for_entry(self, tmp_path: Path) -> None:
+        """Test HMAC computation produces valid hex string."""
+        key_file = tmp_path / "hmac.key"
+        key = b"test_key_12345"
+        key_file.write_bytes(key)
+        entry = {"timestamp": "2025-01-01T00:00:00", "workout_data": {"type": "test"}}
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            result = compute_entry_hmac(entry)
+        assert result is not None
+        # Verify manually
+        payload = json.dumps(entry, sort_keys=True, separators=(",", ":"))
+        expected = hmac.new(key, payload.encode(), hashlib.sha256).hexdigest()
+        assert result == expected
+
+    def test_returns_none_when_no_key(self, tmp_path: Path) -> None:
+        """Test returns None when key file is missing."""
+        key_file = tmp_path / "nonexistent.key"
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            result = compute_entry_hmac({"data": "test"})
+        assert result is None
+
+
+class TestVerifyEntryHmac:
+    """Tests for verify_entry_hmac."""
+
+    def test_valid_hmac(self, tmp_path: Path) -> None:
+        """Test verification passes with correct HMAC."""
+        key_file = tmp_path / "hmac.key"
+        key = b"verification_key"
+        key_file.write_bytes(key)
+        entry_data = {"timestamp": "2025-01-01", "workout_data": {"type": "test"}}
+        payload = json.dumps(entry_data, sort_keys=True, separators=(",", ":"))
+        correct_hmac = hmac.new(key, payload.encode(), hashlib.sha256).hexdigest()
+        entry = {**entry_data, "hmac": correct_hmac}
+
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            assert verify_entry_hmac(entry) is True
+
+    def test_invalid_hmac(self, tmp_path: Path) -> None:
+        """Test verification fails with wrong HMAC."""
+        key_file = tmp_path / "hmac.key"
+        key_file.write_bytes(b"verification_key")
+        entry = {"timestamp": "2025-01-01", "hmac": "wrong_hmac_value"}
+
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            assert verify_entry_hmac(entry) is False
+
+    def test_missing_hmac_field(self) -> None:
+        """Test verification fails when entry has no hmac field."""
+        entry: dict[str, object] = {"timestamp": "2025-01-01"}
+        assert verify_entry_hmac(entry) is False
+
+    def test_non_string_hmac_field(self) -> None:
+        """Test verification fails when hmac field is not a string."""
+        entry: dict[str, object] = {"timestamp": "2025-01-01", "hmac": 12345}
+        assert verify_entry_hmac(entry) is False
+
+    def test_missing_key_file(self, tmp_path: Path) -> None:
+        """Test verification fails when key file doesn't exist."""
+        key_file = tmp_path / "nonexistent.key"
+        entry = {"timestamp": "2025-01-01", "hmac": "some_hmac"}
+        with patch(
+            "python_pkg.screen_locker._log_integrity.HMAC_KEY_FILE",
+            key_file,
+        ):
+            assert verify_entry_hmac(entry) is False
diff --git a/python_pkg/screen_locker/tests/test_phone_check_unlock.py b/python_pkg/screen_locker/tests/test_phone_check_unlock.py
index 08582ef..7ac871c 100644
--- a/python_pkg/screen_locker/tests/test_phone_check_unlock.py
+++ b/python_pkg/screen_locker/tests/test_phone_check_unlock.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 from python_pkg.screen_locker.screen_lock import (
     PHONE_PENALTY_DELAY_DEMO,
@@ -24,34 +24,49 @@ class TestVerifyPhoneWorkout:
         mock_sys_exit: MagicMock,
         tmp_path: Path,
     ) -> None:
-        """Test workout verified on phone."""
+        """Test workout verified on phone with sufficient duration."""
         locker = create_locker(mock_tk, tmp_path)
         object.__setattr__(
             locker,
             "_is_phone_connected",
-            MagicMock(
-                return_value=True,
-            ),
+            MagicMock(return_value=True),
         )
         object.__setattr__(
             locker,
             "_pull_stronglifts_db",
-            MagicMock(
-                return_value=tmp_path / "sl.db",
-            ),
+            MagicMock(return_value=tmp_path / "sl.db"),
         )
         object.__setattr__(
             locker,
             "_count_today_workouts",
-            MagicMock(
-                return_value=2,
-            ),
+            MagicMock(return_value=2),
+        )
+        object.__setattr__(
+            locker,
+            "_is_workout_finish_recent",
+            MagicMock(return_value=True),
+        )
+        object.__setattr__(
+            locker,
+            "_get_today_exercise_count",
+            MagicMock(return_value=3),
+        )
+        object.__setattr__(
+            locker,
+            "_get_today_workout_duration_minutes",
+            MagicMock(return_value=65.0),
         )
 
-        status, message = locker._verify_phone_workout()
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(True, "Clock OK"),
+        ):
+            status, message = locker._verify_phone_workout()
 
         assert status == "verified"
         assert "2 session" in message
+        assert "65 min" in message
+        assert "3 exercise" in message
 
     def test_not_verified(
         self,
@@ -64,30 +79,77 @@ class TestVerifyPhoneWorkout:
         object.__setattr__(
             locker,
             "_is_phone_connected",
-            MagicMock(
-                return_value=True,
-            ),
+            MagicMock(return_value=True),
         )
         object.__setattr__(
             locker,
             "_pull_stronglifts_db",
-            MagicMock(
-                return_value=tmp_path / "sl.db",
-            ),
+            MagicMock(return_value=tmp_path / "sl.db"),
         )
         object.__setattr__(
             locker,
             "_count_today_workouts",
-            MagicMock(
-                return_value=0,
-            ),
+            MagicMock(return_value=0),
         )
 
-        status, message = locker._verify_phone_workout()
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(True, "Clock OK"),
+        ):
+            status, message = locker._verify_phone_workout()
 
         assert status == "not_verified"
         assert "No workout" in message
 
+    def test_too_short(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test workout found but too short."""
+        locker = create_locker(mock_tk, tmp_path)
+        object.__setattr__(
+            locker,
+            "_is_phone_connected",
+            MagicMock(return_value=True),
+        )
+        object.__setattr__(
+            locker,
+            "_pull_stronglifts_db",
+            MagicMock(return_value=tmp_path / "sl.db"),
+        )
+        object.__setattr__(
+            locker,
+            "_count_today_workouts",
+            MagicMock(return_value=1),
+        )
+        object.__setattr__(
+            locker,
+            "_is_workout_finish_recent",
+            MagicMock(return_value=True),
+        )
+        object.__setattr__(
+            locker,
+            "_get_today_exercise_count",
+            MagicMock(return_value=3),
+        )
+        object.__setattr__(
+            locker,
+            "_get_today_workout_duration_minutes",
+            MagicMock(return_value=25.0),
+        )
+
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(True, "Clock OK"),
+        ):
+            status, message = locker._verify_phone_workout()
+
+        assert status == "too_short"
+        assert "25 min" in message
+        assert "50 min" in message
+
     def test_no_phone(
         self,
         mock_tk: MagicMock,
@@ -99,12 +161,14 @@ class TestVerifyPhoneWorkout:
         object.__setattr__(
             locker,
             "_is_phone_connected",
-            MagicMock(
-                return_value=False,
-            ),
+            MagicMock(return_value=False),
         )
 
-        status, _ = locker._verify_phone_workout()
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(True, "Clock OK"),
+        ):
+            status, _ = locker._verify_phone_workout()
 
         assert status == "no_phone"
 
@@ -119,23 +183,122 @@ class TestVerifyPhoneWorkout:
         object.__setattr__(
             locker,
             "_is_phone_connected",
-            MagicMock(
-                return_value=True,
-            ),
+            MagicMock(return_value=True),
         )
         object.__setattr__(
             locker,
             "_pull_stronglifts_db",
-            MagicMock(
-                return_value=None,
-            ),
+            MagicMock(return_value=None),
         )
 
-        status, message = locker._verify_phone_workout()
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(True, "Clock OK"),
+        ):
+            status, message = locker._verify_phone_workout()
 
         assert status == "error"
         assert "database" in message.lower()
 
+    def test_clock_tampered(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test clock_tampered when NTP check fails."""
+        locker = create_locker(mock_tk, tmp_path)
+
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(False, "System clock is 600s ahead"),
+        ):
+            status, message = locker._verify_phone_workout()
+
+        assert status == "clock_tampered"
+        assert "600s" in message
+
+    def test_stale_workout(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test stale status when workout finish is not recent."""
+        locker = create_locker(mock_tk, tmp_path)
+        object.__setattr__(
+            locker,
+            "_is_phone_connected",
+            MagicMock(return_value=True),
+        )
+        object.__setattr__(
+            locker,
+            "_pull_stronglifts_db",
+            MagicMock(return_value=tmp_path / "sl.db"),
+        )
+        object.__setattr__(
+            locker,
+            "_count_today_workouts",
+            MagicMock(return_value=1),
+        )
+        object.__setattr__(
+            locker,
+            "_is_workout_finish_recent",
+            MagicMock(return_value=False),
+        )
+
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(True, "Clock OK"),
+        ):
+            status, message = locker._verify_phone_workout()
+
+        assert status == "stale"
+        assert "old" in message.lower()
+
+    def test_no_exercises(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test no_exercises when workout has no exercise data."""
+        locker = create_locker(mock_tk, tmp_path)
+        object.__setattr__(
+            locker,
+            "_is_phone_connected",
+            MagicMock(return_value=True),
+        )
+        object.__setattr__(
+            locker,
+            "_pull_stronglifts_db",
+            MagicMock(return_value=tmp_path / "sl.db"),
+        )
+        object.__setattr__(
+            locker,
+            "_count_today_workouts",
+            MagicMock(return_value=1),
+        )
+        object.__setattr__(
+            locker,
+            "_is_workout_finish_recent",
+            MagicMock(return_value=True),
+        )
+        object.__setattr__(
+            locker,
+            "_get_today_exercise_count",
+            MagicMock(return_value=0),
+        )
+
+        with patch(
+            "python_pkg.screen_locker._phone_verification.check_clock_skew",
+            return_value=(True, "Clock OK"),
+        ):
+            status, message = locker._verify_phone_workout()
+
+        assert status == "no_exercises"
+        assert "exercise" in message.lower()
+
 
 class TestStartPhoneCheck:
     """Tests for _start_phone_check and _handle_startup_phone_result."""
@@ -197,6 +360,71 @@ class TestStartPhoneCheck:
 
         locker._show_retry_and_sick.assert_called_once()
 
+    def test_handle_startup_too_short_shows_retry_and_sick(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test too_short result shows retry and sick buttons."""
+        locker = create_locker(mock_tk, tmp_path)
+        object.__setattr__(locker, "_show_retry_and_sick", MagicMock())
+        locker._handle_startup_phone_result(
+            "too_short", "Workout too short! 25 min logged, need at least 50 min."
+        )
+
+        locker._show_retry_and_sick.assert_called_once()
+        call_args = locker._show_retry_and_sick.call_args[0][0]
+        assert "too short" in call_args.lower()
+
+    def test_handle_startup_stale_shows_retry_and_sick(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test stale result shows retry and sick buttons."""
+        locker = create_locker(mock_tk, tmp_path)
+        object.__setattr__(locker, "_show_retry_and_sick", MagicMock())
+        locker._handle_startup_phone_result("stale", "Workout too old")
+
+        locker._show_retry_and_sick.assert_called_once()
+        call_args = locker._show_retry_and_sick.call_args[0][0]
+        assert "suspicious" in call_args.lower()
+
+    def test_handle_startup_no_exercises_shows_retry_and_sick(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test no_exercises result shows retry and sick buttons."""
+        locker = create_locker(mock_tk, tmp_path)
+        object.__setattr__(locker, "_show_retry_and_sick", MagicMock())
+        locker._handle_startup_phone_result("no_exercises", "No exercises found")
+
+        locker._show_retry_and_sick.assert_called_once()
+        call_args = locker._show_retry_and_sick.call_args[0][0]
+        assert "suspicious" in call_args.lower()
+
+    def test_handle_startup_clock_tampered_shows_retry_and_sick(
+        self,
+        mock_tk: MagicMock,
+        mock_sys_exit: MagicMock,
+        tmp_path: Path,
+    ) -> None:
+        """Test clock_tampered result shows retry and sick buttons."""
+        locker = create_locker(mock_tk, tmp_path)
+        object.__setattr__(locker, "_show_retry_and_sick", MagicMock())
+        locker._handle_startup_phone_result(
+            "clock_tampered",
+            "System clock is 600s ahead",
+        )
+
+        locker._show_retry_and_sick.assert_called_once()
+        call_args = locker._show_retry_and_sick.call_args[0][0]
+        assert "clock" in call_args.lower()
+
     def test_handle_startup_no_phone_shows_penalty(
         self,
         mock_tk: MagicMock,
diff --git a/python_pkg/screen_locker/tests/test_time_check.py b/python_pkg/screen_locker/tests/test_time_check.py
new file mode 100644
index 0000000..b8474bc
--- /dev/null
+++ b/python_pkg/screen_locker/tests/test_time_check.py
@@ -0,0 +1,108 @@
+"""Tests for _time_check NTP clock skew detection."""
+
+from __future__ import annotations
+
+import struct
+import time
+from unittest.mock import MagicMock, patch
+
+from python_pkg.screen_locker._time_check import (
+    _NTP_EPOCH_OFFSET,
+    _query_ntp_offset,
+    check_clock_skew,
+)
+
+
+class TestQueryNtpOffset:
+    """Tests for _query_ntp_offset."""
+
+    def test_returns_offset_on_success(self) -> None:
+        """Test returns float offset when NTP server responds."""
+        now = time.time()
+        # Build a fake NTP response with server time close to now
+        server_ntp = int(now + _NTP_EPOCH_OFFSET)
+        fraction = 0
+        response = b"\x00" * 40 + struct.pack("!II", server_ntp, fraction)
+
+        mock_socket = MagicMock()
+        mock_socket.__enter__ = MagicMock(return_value=mock_socket)
+        mock_socket.__exit__ = MagicMock(return_value=False)
+        mock_socket.recvfrom.return_value = (response, ("pool.ntp.org", 123))
+
+        with patch("socket.socket", return_value=mock_socket):
+            offset = _query_ntp_offset()
+
+        assert offset is not None
+        assert abs(offset) < 5  # Should be very close to zero
+
+    def test_returns_none_on_oserror(self) -> None:
+        """Test returns None when socket fails."""
+        mock_socket = MagicMock()
+        mock_socket.__enter__ = MagicMock(return_value=mock_socket)
+        mock_socket.__exit__ = MagicMock(return_value=False)
+        mock_socket.sendto.side_effect = OSError("network unreachable")
+
+        with patch("socket.socket", return_value=mock_socket):
+            offset = _query_ntp_offset()
+
+        assert offset is None
+
+    def test_returns_none_on_short_response(self) -> None:
+        """Test returns None when NTP response is too short."""
+        mock_socket = MagicMock()
+        mock_socket.__enter__ = MagicMock(return_value=mock_socket)
+        mock_socket.__exit__ = MagicMock(return_value=False)
+        mock_socket.recvfrom.return_value = (b"\x00" * 10, ("pool.ntp.org", 123))
+
+        with patch("socket.socket", return_value=mock_socket):
+            offset = _query_ntp_offset()
+
+        assert offset is None
+
+
+class TestCheckClockSkew:
+    """Tests for check_clock_skew."""
+
+    def test_ok_within_threshold(self) -> None:
+        """Test returns ok when clock offset is small."""
+        with patch(
+            "python_pkg.screen_locker._time_check._query_ntp_offset",
+            return_value=2.5,
+        ):
+            ok, message = check_clock_skew()
+
+        assert ok is True
+        assert "OK" in message
+
+    def test_fails_when_skew_exceeds_threshold(self) -> None:
+        """Test returns failure when clock offset exceeds max."""
+        with patch(
+            "python_pkg.screen_locker._time_check._query_ntp_offset",
+            return_value=600.0,
+        ):
+            ok, message = check_clock_skew()
+
+        assert ok is False
+        assert "600" in message
+
+    def test_ntp_unreachable_passes(self) -> None:
+        """Test returns ok when NTP server is unreachable (fail-open)."""
+        with patch(
+            "python_pkg.screen_locker._time_check._query_ntp_offset",
+            return_value=None,
+        ):
+            ok, message = check_clock_skew()
+
+        assert ok is True
+        assert "skipped" in message.lower()
+
+    def test_negative_offset_detected(self) -> None:
+        """Test detects clock ahead with negative offset."""
+        with patch(
+            "python_pkg.screen_locker._time_check._query_ntp_offset",
+            return_value=-400.0,
+        ):
+            ok, message = check_clock_skew()
+
+        assert ok is False
+        assert "ahead" in message.lower()
diff --git a/python_pkg/screen_locker/workout-locker.service b/python_pkg/screen_locker/workout-locker.service
index 2d76902..47e5437 100644
--- a/python_pkg/screen_locker/workout-locker.service
+++ b/python_pkg/screen_locker/workout-locker.service
@@ -7,9 +7,11 @@ Type=simple
 WorkingDirectory=/home/kuhy/testsAndMisc
 Environment=DISPLAY=:0
 Environment=PYTHONPATH=/home/kuhy/testsAndMisc
-ExecStartPre=/bin/sleep 3
+ExecStartPre=/bin/sleep 1
 ExecStart=/usr/bin/python3 -m python_pkg.screen_locker.screen_lock --production
-Restart=no
+Restart=on-failure
+RestartSec=2s
+RestartPreventExitStatus=0
 User=%u
 
 [Install]
diff --git a/python_pkg/screen_locker/workout-locker.timer b/python_pkg/screen_locker/workout-locker.timer
index 7b57a66..4a01809 100644
--- a/python_pkg/screen_locker/workout-locker.timer
+++ b/python_pkg/screen_locker/workout-locker.timer
@@ -2,7 +2,7 @@
 Description=Periodically check if workout was done today
 
 [Timer]
-OnBootSec=30s
+OnBootSec=5s
 OnUnitActiveSec=15min
 Persistent=true