Google Calendar bot: Include time zone offsets in timestamps.

Fixes: #3769

integrations/google/google-calendar

@@ -1,11 +1,16 @@
 #!/usr/bin/env python
+#
+# This script depends on python-dateutil and python-pytz for properly handling
+# times and time zones of calendar events.
 from __future__ import print_function
 import datetime
+import dateutil.parser
 import httplib2
 import itertools
 import logging
 import optparse
 import os
+import pytz
 from six.moves import urllib
 import sys
 import time
@@ -114,13 +119,26 @@ def get_events():
 
     for event in feed["items"]:
         try:
-            start = event["start"]["dateTime"]
+            start = dateutil.parser.parse(event["start"]["dateTime"])
+            # According to the API documentation, a time zone offset is required
+            # for start.dateTime unless a time zone is explicitly specified in
+            # start.timeZone.
+            if start.tzinfo is None:
+                event_timezone = pytz.timezone(event["start"]["timeZone"])
+                # pytz timezones include an extra localize method that's not part
+                # of the tzinfo base class.
+                start = event_timezone.localize(start) # type: ignore
         except KeyError:
-            start = event["start"]["date"]
-        start = start[:19]
-        # All-day events can have only a date
-        fmt = '%Y-%m-%dT%H:%M:%S' if 'T' in start else '%Y-%m-%d'
-        start = datetime.datetime.strptime(start, fmt)
+            # All-day events can have only a date.
+            start_naive = dateutil.parser.parse(event["start"]["date"])
+
+            # All-day events don't have a time zone offset; instead, we use the
+            # time zone of the calendar.
+            calendar_timezone = pytz.timezone(feed["timeZone"])
+            # pytz timezones include an extra localize method that's not part
+            # of the tzinfo base class.
+            start = calendar_timezone.localize(start_naive) # type: ignore
+
         try:
             yield (event["id"], start, event["summary"])
         except KeyError:
@@ -133,7 +151,7 @@ def send_reminders():
 
     messages = []
     keys = set()
-    now = datetime.datetime.now()
+    now = datetime.datetime.now(tz=pytz.utc)
 
     for id, start, summary in events:
         dt = start - now