find DST transitions to the second

`get_dst_transitions` probed the offset once every 24 hours from the start time and reported a change at the first probe after it, up to a day late, so events, review items and recordings near a transition were grouped into days with the old offset. A transition after the last daily probe wasn't found at all. The end of the range is probed too now, and a probe that sees the offset change bisects the interval to the second of the transition.

frigate/test/test_dst_transitions.py

@@ -0,0 +1,54 @@
+"""Tests for get_dst_transitions."""
+
+import datetime
+import unittest
+
+from frigate.util.time import get_dst_transitions
+
+
+class TestDstTransitions(unittest.TestCase):
+    def test_dst_transition_splits_periods_at_the_transition(self):
+        start = datetime.datetime(2026, 3, 7, 12, tzinfo=datetime.UTC).timestamp()
+        end = start + 2 * 86400
+        spring = datetime.datetime(2026, 3, 8, 7, tzinfo=datetime.UTC).timestamp()
+        self.assertEqual(
+            get_dst_transitions("America/New_York", start, end),
+            [(start, spring, -18000), (spring, end, -14400)],
+        )
+
+    def test_dst_transition_is_not_reported_a_day_late(self):
+        # local midnight on the day of the change used to report the
+        # transition a full day after it actually happened
+        start = datetime.datetime(2024, 3, 10, 5, tzinfo=datetime.UTC).timestamp()
+        end = start + 3 * 86400
+        spring = datetime.datetime(2024, 3, 10, 7, tzinfo=datetime.UTC).timestamp()
+        self.assertEqual(
+            get_dst_transitions("America/New_York", start, end),
+            [(start, spring, -18000), (spring, end, -14400)],
+        )
+
+    def test_dst_transition_after_the_last_daily_probe_is_found(self):
+        start = datetime.datetime(2024, 11, 2, 12, tzinfo=datetime.UTC).timestamp()
+        end = datetime.datetime(2024, 11, 3, 10, tzinfo=datetime.UTC).timestamp()
+        fall = datetime.datetime(2024, 11, 3, 6, tzinfo=datetime.UTC).timestamp()
+        self.assertEqual(
+            get_dst_transitions("America/New_York", start, end),
+            [(start, fall, -14400), (fall, end, -18000)],
+        )
+
+    def test_no_transition_returns_a_single_period(self):
+        start = datetime.datetime(2026, 6, 1, tzinfo=datetime.UTC).timestamp()
+        end = start + 5 * 86400
+        self.assertEqual(
+            get_dst_transitions("America/New_York", start, end),
+            [(start, end, -14400)],
+        )
+
+    def test_invalid_zone_retains_utc_fallback(self):
+        self.assertEqual(
+            get_dst_transitions("Invalid/Timezone", 100, 200), [(100, 200, 0)]
+        )
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)

frigate/util/time.py

@@ -2,6 +2,7 @@
 
 import datetime
 import logging
+import math
 from zoneinfo import ZoneInfoNotFoundError
 
 import pytz
@@ -43,9 +44,33 @@ def is_current_hour(timestamp: int) -> bool:
     return timestamp < start_of_next_hour
 
 
+def _utc_offset(tz: datetime.tzinfo, timestamp: float) -> float:
+    dt = datetime.datetime.fromtimestamp(timestamp, tz=datetime.UTC)
+    return dt.astimezone(tz).utcoffset().total_seconds()
+
+
+def _find_transition(
+    tz: datetime.tzinfo, lo: float, hi: float, lo_offset: float
+) -> float:
+    """Bisect (lo, hi] to the second where the UTC offset first differs from lo_offset."""
+    # whole seconds, so the midpoint always advances (a fractional bound can
+    # otherwise leave the midpoint sitting on lo) and lands on the transition
+    low = math.floor(lo)
+    high = math.ceil(hi)
+
+    while high - low > 1:
+        mid = (low + high) // 2
+        if _utc_offset(tz, mid) == lo_offset:
+            low = mid
+        else:
+            high = mid
+
+    return float(high)
+
+
 def get_dst_transitions(
     tz_name: str, start_time: float, end_time: float
-) -> list[tuple[float, float]]:
+) -> list[tuple[float, float, float]]:
     """
     Find DST transition points and return time periods with consistent offsets.
 
@@ -66,28 +91,24 @@ def get_dst_transitions(
 
     periods = []
     current = start_time
-
-    # Get initial offset
-    dt = datetime.datetime.utcfromtimestamp(current).replace(tzinfo=pytz.UTC)
-    local_dt = dt.astimezone(tz)
-    prev_offset = local_dt.utcoffset().total_seconds()
     period_start = start_time
+    prev_offset = _utc_offset(tz, current)
 
-    # Check each day for offset changes
-    while current <= end_time:
-        dt = datetime.datetime.utcfromtimestamp(current).replace(tzinfo=pytz.UTC)
-        local_dt = dt.astimezone(tz)
-        current_offset = local_dt.utcoffset().total_seconds()
+    # Probe at most a day ahead, capped at end_time so a transition after the
+    # last full day is still seen instead of silently kept in the last period.
+    while current < end_time:
+        next_probe = min(current + 86400, end_time)
+        next_offset = _utc_offset(tz, next_probe)
 
-        if current_offset != prev_offset:
-            # Found a transition - close previous period
-            periods.append((period_start, current, prev_offset))
-            period_start = current
-            prev_offset = current_offset
+        if next_offset != prev_offset:
+            transition = _find_transition(tz, current, next_probe, prev_offset)
+            periods.append((period_start, transition, prev_offset))
+            period_start = transition
+            prev_offset = _utc_offset(tz, transition)
+            current = transition
+        else:
+            current = next_probe
 
-        current += 86400  # Check daily
-
-    # Add final period
     periods.append((period_start, end_time, prev_offset))
 
     return periods