Fix ARG_NOT_SET deserialization for _date fields in OperatorSerialization

airflow-core/src/airflow/serialization/serialized_objects.py

@@ -1587,6 +1587,14 @@ def _deserialize_field_value(cls, field_name: str, value: Any) -> Any:
         elif field_name == "resources":
             return Resources.from_dict(value) if value is not None else None
         elif field_name.endswith("_date"):
+            # Handle ARG_NOT_SET (NOTSET singleton) before trying to parse as datetime.
+            # Without this, deserializing operators that store a date field as ``NOTSET``
+            # (e.g. ``TriggerDagRunOperator.logical_date``) would fail because
+            # ``_deserialize_datetime`` does not understand the ARG_NOT_SET encoding.
+            if isinstance(value, dict) and value.get(Encoding.TYPE) == DAT.ARG_NOT_SET:
+                from airflow.serialization.definitions.notset import NOTSET
+
+                return NOTSET
             return cls._deserialize_datetime(value) if value is not None else None
         else:
             # For all other fields, return as-is (strings, ints, bools, etc.)

airflow-core/tests/unit/serialization/test_serialized_objects.py

@@ -1011,6 +1011,23 @@ def test_deserialize_datetime_with_timestamp(self):
         assert isinstance(result, datetime)
         assert result.timestamp() == timestamp
 
+    def test_deserialize_field_value_with_arg_not_set_for_date_fields(self):
+        """``_deserialize_field_value`` returns ``NOTSET`` for ARG_NOT_SET date fields.
+
+        Operators may store ``NOTSET`` (an ``ArgNotSet`` instance) on date-suffixed
+        fields such as ``logical_date`` to mean "use the default at runtime". When
+        such a field is round-tripped through serialization, the ARG_NOT_SET
+        encoding must be restored to ``NOTSET`` instead of being parsed as a
+        datetime.
+        """
+        from airflow.serialization.definitions.notset import NOTSET
+        from airflow.serialization.serialized_objects import OperatorSerialization
+
+        value = {Encoding.TYPE: DAT.ARG_NOT_SET}
+
+        for field_name in ("logical_date", "start_date", "end_date"):
+            assert OperatorSerialization._deserialize_field_value(field_name, value) is NOTSET
+
 
 class TestRetryPolicySerialization:
     """Test that retry_policy is serialized as a boolean flag (has_retry_policy)."""