diff --git a/api/db/db_models.py b/api/db/db_models.py
index ce9406cc3f..3867612210 100644
--- a/api/db/db_models.py
+++ b/api/db/db_models.py
@@ -1316,15 +1316,24 @@ class DateTimeTzField(CharField):
                 return value.replace(tzinfo=timezone.utc).isoformat()
         return value
 
-    def python_value(self, value: str | None) -> datetime | None:
-        if value is not None:
+    def python_value(self, value: str | datetime | None) -> datetime | None:
+        if value is None:
+            return None
+        if isinstance(value, datetime):
+            # The column is declared VARCHAR, but deployments upgraded from
+            # older schemas may hold it as native DATETIME, in which case the
+            # driver returns datetime objects instead of ISO strings.
+            return value if value.tzinfo is not None else value.replace(tzinfo=timezone.utc)
+        try:
             dt = datetime.fromisoformat(value)
-            if dt.tzinfo is None:
-                import pytz
-
-                return dt.replace(tzinfo=pytz.UTC)
-            return dt
-        return value
+        except ValueError:
+            # Zero dates (0000-00-00) and other unparseable values must not
+            # raise here: peewee counts the row before converting it, so one
+            # bad value wedges every later iteration over the table with
+            # IndexError: list index out of range.
+            logging.warning("DateTimeTzField: unparseable value %r, falling back to None", value)
+            return None
+        return dt if dt.tzinfo is not None else dt.replace(tzinfo=timezone.utc)
 
 
 class SyncLogs(DataBaseModel):
diff --git a/test/unit_test/api/db/test_datetime_tz_field.py b/test/unit_test/api/db/test_datetime_tz_field.py
new file mode 100644
index 0000000000..427c14a308
--- /dev/null
+++ b/test/unit_test/api/db/test_datetime_tz_field.py
@@ -0,0 +1,62 @@
+"""
+Tests for DateTimeTzField conversion between database driver values and
+Python datetimes.
+
+The sync_logs poll_range columns are declared VARCHAR, but deployments
+upgraded from older schemas can hold them in native DATETIME columns, so the
+driver may hand back datetime objects (or zero-date strings) instead of ISO
+strings. Conversion must never raise: one bad value previously wedged the
+whole sync-task scheduler with "list index out of range".
+"""
+
+from datetime import UTC, datetime
+
+from api.db.db_models import DateTimeTzField
+
+
+class TestDateTimeTzFieldPythonValue:
+    def test_none(self):
+        assert DateTimeTzField().python_value(None) is None
+
+    def test_iso_string_with_timezone(self):
+        dt = DateTimeTzField().python_value("2026-08-14T10:00:00.123456+00:00")
+        assert dt == datetime(2026, 8, 14, 10, 0, 0, 123456, tzinfo=UTC)
+
+    def test_iso_string_without_timezone_assumes_utc(self):
+        dt = DateTimeTzField().python_value("2026-08-14 10:00:00")
+        assert dt == datetime(2026, 8, 14, 10, 0, tzinfo=UTC)
+
+    def test_native_datetime_returned_by_driver(self):
+        naive = datetime(2026, 8, 14, 10, 0, 0, 123000, tzinfo=UTC).replace(tzinfo=None)
+        dt = DateTimeTzField().python_value(naive)
+        assert dt == datetime(2026, 8, 14, 10, 0, 0, 123000, tzinfo=UTC)
+
+    def test_native_aware_datetime_kept(self):
+        aware = datetime(2026, 8, 14, 10, 0, tzinfo=UTC)
+        assert DateTimeTzField().python_value(aware) is aware
+
+    def test_zero_date_string_falls_back_to_none(self):
+        assert DateTimeTzField().python_value("0000-00-00 00:00:00") is None
+
+    def test_garbage_string_falls_back_to_none(self):
+        assert DateTimeTzField().python_value("not-a-date") is None
+
+    def test_empty_string_falls_back_to_none(self):
+        assert DateTimeTzField().python_value("") is None
+
+
+class TestDateTimeTzFieldDbValue:
+    def test_none(self):
+        assert DateTimeTzField().db_value(None) is None
+
+    def test_naive_datetime_written_as_utc_iso(self):
+        naive = datetime(2026, 8, 14, 10, 0, tzinfo=UTC).replace(tzinfo=None)
+        assert DateTimeTzField().db_value(naive) == "2026-08-14T10:00:00+00:00"
+
+    def test_aware_datetime_written_as_iso(self):
+        assert DateTimeTzField().db_value(datetime(2026, 8, 14, 10, 0, tzinfo=UTC)) == "2026-08-14T10:00:00+00:00"
+
+    def test_db_value_round_trips_through_python_value(self):
+        field = DateTimeTzField()
+        stored = field.db_value(datetime(2026, 8, 14, 10, 0, 0, 123456, tzinfo=UTC))
+        assert field.python_value(stored) == datetime(2026, 8, 14, 10, 0, 0, 123456, tzinfo=UTC)