Fix DateField for GMT+ users (#4371)

* convert to timezone only if datetime is tzaware; add exposing test

fiftyone/core/fields.py

@@ -516,8 +516,14 @@ def to_python(self, value):
 
         # Explicitly converting to UTC is important here because PyMongo loads
         # everything as `datetime`, which will respect `fo.config.timezone`,
-        # but we always need UTC here for the conversion back to `date`
-        return value.astimezone(pytz.utc).date()
+        # but we always need UTC here for the conversion back to `date` because
+        # we write to the DB in UTC time.
+        # If value is timezone unaware, then do not convert because it will be
+        # assumed to be in local timezone and date will be wrong for GMT+
+        # localities.
+        if value.tzinfo is not None:
+            value = value.astimezone(pytz.utc)
+        return value.date()
 
     def validate(self, value):
         if not isinstance(value, date):

fiftyone/core/odm/database.py

@@ -422,7 +422,7 @@ def get_async_db_conn(use_global=False):
 def _apply_options(db):
     timezone = fo.config.timezone
 
-    if not timezone or timezone.lower() == "utc":
+    if not timezone:
         return db
 
     if timezone.lower() == "local":

tests/unittests/dataset_tests.py

@@ -5,6 +5,7 @@
 | `voxel51.com <https://voxel51.com/>`_
 |
 """
+import time
 from copy import deepcopy, copy
 from datetime import date, datetime, timedelta
 import gc
@@ -558,6 +559,22 @@ def test_date_fields(self):
         self.assertEqual(type(sample.date), date)
         self.assertEqual(int((sample.date - date1).total_seconds()), 0)
 
+        # Now change system time to something GMT+
+        system_timezone = os.environ.get("TZ")
+        try:
+            os.environ["TZ"] = "Europe/Madrid"
+            time.tzset()
+            dataset.reload()
+        finally:
+            if system_timezone is None:
+                del os.environ["TZ"]
+            else:
+                os.environ["TZ"] = system_timezone
+            time.tzset()
+
+        self.assertEqual(type(sample.date), date)
+        self.assertEqual(int((sample.date - date1).total_seconds()), 0)
+
     @drop_datasets
     def test_datetime_fields(self):
         dataset = fo.Dataset()