Fixed failing test around DST change.

The timezone arithmetic done in JS can be off by one hour around DST
change. We work around this issue by adding one extra hour to the test
error margin when we detect a DST change is near.

Refs #20663.

tests/admin_widgets/tests.py

@@ -2,7 +2,12 @@
 from __future__ import unicode_literals
 
 from datetime import datetime, timedelta
-from unittest import TestCase
+from unittest import TestCase, skipIf
+
+try:
+    import pytz
+except ImportError:
+    pytz = None
 
 from django import forms
 from django.conf import settings
@@ -635,6 +640,7 @@ class DateTimePickerSeleniumIETests(DateTimePickerSeleniumFirefoxTests):
     webdriver_class = 'selenium.webdriver.ie.webdriver.WebDriver'
 
 
+@skipIf(pytz is None, "this test requires pytz")
 @override_settings(TIME_ZONE='Asia/Singapore')
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))
 class DateTimePickerShortcutsSeleniumFirefoxTests(AdminSeleniumWebDriverTestCase):
@@ -654,9 +660,18 @@ class DateTimePickerShortcutsSeleniumFirefoxTests(AdminSeleniumWebDriverTestCase
         """
         self.admin_login(username='super', password='secret', login_url='/')
 
-        now = datetime.now()
         error_margin = timedelta(seconds=10)
 
+        # If we are neighbouring a DST, we add an hour of error margin.
+        tz = pytz.timezone('America/Chicago')
+        utc_now = datetime.now(pytz.utc)
+        tz_yesterday = (utc_now - timedelta(days=1)).astimezone(tz).tzname()
+        tz_tomorrow = (utc_now + timedelta(days=1)).astimezone(tz).tzname()
+        if tz_yesterday != tz_tomorrow:
+            error_margin += timedelta(hours=1)
+
+        now = datetime.now()
+
         self.selenium.get('%s%s' % (self.live_server_url,
             '/admin_widgets/member/add/'))