Revert "Amend tests for changes is timestamp formatting"

This reverts commit 49c2523.

parkings/tests/api/enforcement/test_valid_parking.py

@@ -2,7 +2,7 @@
 
 import pytest
 from django.core.urlresolvers import reverse
-from django.utils.timezone import localtime, utc
+from django.utils.timezone import utc
 from rest_framework.status import HTTP_400_BAD_REQUEST, HTTP_401_UNAUTHORIZED, HTTP_403_FORBIDDEN
 
 from ..utils import (
@@ -97,17 +97,11 @@ def check_parking_data_matches_parking_object(parking_data, parking_obj):
 
 
 def iso8601(dt):
-    return add_colon_to_tz(localtime(dt).strftime('%Y-%m-%dT%H:%M:%S%z'))
+    return dt.strftime('%Y-%m-%dT%H:%M:%SZ')
 
 
 def iso8601_us(dt):
-    return add_colon_to_tz(localtime(dt).strftime('%Y-%m-%dT%H:%M:%S.%f%z'))
-
-
-def add_colon_to_tz(timestamp_string):
-    assert isinstance(timestamp_string, str)
-    assert timestamp_string[-4:].isdigit(), "Last 4 chars should be digits"
-    return timestamp_string[:-2] + ':' + timestamp_string[-2:]
+    return dt.strftime('%Y-%m-%dT%H:%M:%S.%fZ')
 
 
 def test_registration_number_filter(operator, staff_api_client, parking_factory):