Fixed django#19929 -- Improved error when MySQL doesn't have TZ definitions.

Thanks tomas_00 for the report.

Author: aaugustin

django/db/models/sql/compiler.py

@@ -1057,6 +1057,10 @@ def results_iter(self):
  # Datetimes are artifically returned in UTC on databases that
  # don't support time zone. Restore the zone used in the query.
  if settings.USE_TZ:
+ if datetime is None:
+ raise ValueError("Database returned an invalid value "
+ "in QuerySet.dates(). Are time zone "
+ "definitions installed?")
  datetime = datetime.replace(tzinfo=None)
  datetime = timezone.make_aware(datetime, self.query.tzinfo)
  yield datetime

docs/ref/contrib/admin/index.txt

@@ -142,6 +142,13 @@ subclass::
  e.g. if all the dates are in one month, it'll show the day-level
  drill-down only.
 
+ .. note::
+
+ ``date_hierarchy`` uses :meth:`QuerySet.datetimes()
+ <django.db.models.query.QuerySet.datetimes>` internally. Please refer
+ to its documentation for some caveats when time zone support is
+ enabled (:setting:`USE_TZ = True <USE_TZ>`).
+
 .. attribute:: ModelAdmin.exclude
 
  This attribute, if given, should be a list of field names to exclude from