Add a setting to set a default timezone for datetime representations

DRF, by default, uses Django's timezone settings for deciding on how to represent datetimes if timezone support is active.
This setting allows the user to instead point to a different timezone for their API endpoint in particular.

docs/api-guide/settings.md

@@ -306,6 +306,14 @@ May be a list including the string `'iso-8601'` or Python [strftime format][strf
 
 Default: `['iso-8601']`
 
+#### DATETIME_TZ
+
+If set, used as the canonical timezone for `DateTimeField` serialization. If this is not set but Django timezone support is active, Django's timezone is instead used
+
+May be any of `None`, or a `datetime.tzinfo` object (such as `pytz.timezone('Asia/Kolkata')`)
+
+Default: None
+
 #### DATE_FORMAT
 
 A format string that should be used by default for rendering the output of `DateField` serializer fields.  If `None`, then `DateField` serializer fields will return Python `date` objects, and the date encoding will be determined by the renderer.

rest_framework/fields.py

@@ -1174,7 +1174,13 @@ class DateTimeField(Field):
         return value
 
     def default_timezone(self):
-        return timezone.get_current_timezone() if settings.USE_TZ else None
+        if settings.USE_TZ:
+            if api_settings.DATETIME_TZ:
+                return api_settings.DATETIME_TZ
+            else:
+                return timezone.get_current_timezone()
+        else:
+            return None
 
     def to_internal_value(self, value):
         input_formats = getattr(self, 'input_formats', api_settings.DATETIME_INPUT_FORMATS)

rest_framework/settings.py

@@ -107,6 +107,7 @@ DEFAULTS = {
 
     'DATETIME_FORMAT': ISO_8601,
     'DATETIME_INPUT_FORMATS': (ISO_8601,),
+    'DATETIME_TZ': None,
 
     'TIME_FORMAT': ISO_8601,
     'TIME_INPUT_FORMATS': (ISO_8601,),

tests/test_fields.py

@@ -14,6 +14,7 @@ from django.utils.timezone import activate, deactivate, utc
 import rest_framework
 from rest_framework import compat, serializers
 from rest_framework.fields import DjangoImageField, is_simple_callable
+from rest_framework.settings import api_settings
 
 try:
     import pytz
@@ -1273,6 +1274,42 @@ class TestTZWithDateTimeField(FieldValues):
         cls.field = serializers.DateTimeField(default_timezone=kolkata)
 
 
+@pytest.mark.skipif(pytz is None, reason='pytz not installed')
+@override_settings(TIME_ZONE='Asia/Kolkata', USE_TZ=True)
+class TestDateTimeWithOverrideField(FieldValues, TestCase):
+    """
+    Valid and invalid values for `DateTimeField` when not using UTC as the timezone.
+    """
+    @classmethod
+    def setup_class(cls):
+        # use class setup method, as class-level attribute will still be evaluated even if test is skipped
+        kolkata = pytz.timezone('Asia/Kolkata')
+        paris = pytz.timezone('Europe/Paris')
+        # set the timezone to a specific timezone that isn't the django-level setting
+        cls.old_setting = api_settings.DATETIME_TZ
+
+        api_settings.DATETIME_TZ = paris
+
+
+        cls.valid_inputs = {
+            '2016-12-19T10:00:00': paris.localize(datetime.datetime(2016, 12, 19, 10)),
+            # datetimes are reworked into the paris tz
+            '2016-12-19T10:00:00+05:30': paris.localize(datetime.datetime(2016, 12, 19, 5, 30)),
+            # naive datetimes are assumed to be in the paris tz
+            datetime.datetime(2016, 12, 19, 10): paris.localize(datetime.datetime(2016, 12, 19, 10)),
+        }
+        cls.invalid_inputs = {}
+        cls.outputs = {
+            datetime.datetime(2016, 12, 19, 10): '2016-12-19T10:00:00+01:00',
+            datetime.datetime(2016, 12, 19, 4, 30, tzinfo=utc): '2016-12-19T05:30:00+01:00',
+        }
+        cls.field = serializers.DateTimeField()
+
+    @classmethod
+    def teardown_class(cls):
+        api_settings.DATETIME_TZ = cls.old_setting
+
+
 @pytest.mark.skipif(pytz is None, reason='pytz not installed')
 @override_settings(TIME_ZONE='UTC', USE_TZ=True)
 class TestDefaultTZDateTimeField(TestCase):