diff --git a/backend/app/services/garmin_connect_sync.py b/backend/app/services/garmin_connect_sync.py
index cb2e847..4e409a6 100644
--- a/backend/app/services/garmin_connect_sync.py
+++ b/backend/app/services/garmin_connect_sync.py
@@ -133,18 +133,24 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
             continue
 
         # Slow-path dedup: activity imported via bulk export (no garmin_activity_id).
-        # Check by start_time; stamp the ID so future syncs skip it in the fast path.
-        act_start_str = act.get("startTimeLocal") or act.get("startTimeGMT") or ""
+        # Match on the actual start instant (not just the date — two activities on
+        # the same day are distinct), comparing GMT-to-GMT since FIT start_times are
+        # stored in UTC. A small window absorbs sub-second/rounding differences.
+        act_start_str = act.get("startTimeGMT") or ""
         if act_start_str:
             try:
-                from datetime import datetime as _dt
+                from datetime import datetime as _dt, timezone as _tz
                 act_start = _dt.fromisoformat(act_start_str.replace("Z", "+00:00"))
+                if act_start.tzinfo is None:
+                    act_start = act_start.replace(tzinfo=_tz.utc)  # startTimeGMT is UTC
+                window = timedelta(minutes=5)
                 time_match = db.execute(
                     select(Activity).where(
                         Activity.user_id == user_id,
-                        func.date(Activity.start_time) == act_start.date(),
+                        Activity.start_time >= act_start - window,
+                        Activity.start_time <= act_start + window,
                     )
-                ).scalar_one_or_none()
+                ).scalars().first()
                 if time_match:
                     if not time_match.garmin_activity_id:
                         time_match.garmin_activity_id = garmin_id
diff --git a/milevault_export/backend/app/services/garmin_connect_sync.py b/milevault_export/backend/app/services/garmin_connect_sync.py
index 8a2ea8f..b32ce03 100644
--- a/milevault_export/backend/app/services/garmin_connect_sync.py
+++ b/milevault_export/backend/app/services/garmin_connect_sync.py
@@ -127,18 +127,24 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
             continue
 
         # Slow-path dedup: activity imported via bulk export (no garmin_activity_id).
-        # Check by start_time; stamp the ID so future syncs skip it in the fast path.
-        act_start_str = act.get("startTimeLocal") or act.get("startTimeGMT") or ""
+        # Match on the actual start instant (not just the date — two activities on
+        # the same day are distinct), comparing GMT-to-GMT since FIT start_times are
+        # stored in UTC. A small window absorbs sub-second/rounding differences.
+        act_start_str = act.get("startTimeGMT") or ""
         if act_start_str:
             try:
-                from datetime import datetime as _dt
+                from datetime import datetime as _dt, timezone as _tz
                 act_start = _dt.fromisoformat(act_start_str.replace("Z", "+00:00"))
+                if act_start.tzinfo is None:
+                    act_start = act_start.replace(tzinfo=_tz.utc)  # startTimeGMT is UTC
+                window = timedelta(minutes=5)
                 time_match = db.execute(
                     select(Activity).where(
                         Activity.user_id == user_id,
-                        func.date(Activity.start_time) == act_start.date(),
+                        Activity.start_time >= act_start - window,
+                        Activity.start_time <= act_start + window,
                     )
-                ).scalar_one_or_none()
+                ).scalars().first()
                 if time_match:
                     if not time_match.garmin_activity_id:
                         time_match.garmin_activity_id = garmin_id