[Findit] Fix flaky tests due to importing pytz.

appengine/findit/common/time_util.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import calendar
 from datetime import datetime
 from datetime import time
 from datetime import timedelta
 
 import pytz
@@ skipping 27 matching lines @@
   return '%02d:%02d:%02d' % (hours, minutes, seconds)
 
 
 def FormatDatetime(date):
   if not date:
     return None
   else:
     return date.strftime('%Y-%m-%d %H:%M:%S UTC')
 
 
-def GetDatetimeInTimezone(timezone_name, date_time=None):
+def GetDatetimeInTimezone(timezone_name, date_time):
   """Returns the datetime.datetime of the given one in the specified timezone.
 
   Args:
     timezone_name (str): The name of any timezone supported by pytz.
     date_time (datetime.datetime): The optional datetime to be converted into
-        the new timezone. If not given, default to UTC now.
+        the new timezone.
 
   Returns:
     A datetime.datetime of the given one in the specified timezone.
   """
-  date_time = date_time or GetUTCNowWithTimezone()
   return date_time.astimezone(pytz.timezone(timezone_name))