Events: work around time-zone interactions with URLs

The archive URLs are date-coded but because they're based off of a
timezone-aware DateTimeField there was the possibility of a 404 any time
the start_time was close enough the date boundary of the user's
timezone (see https://code.djangoproject.com/ticket/18794)

Since we want to use our calendar dates for the archive, this change
simply overrides the check to use only the year/month/day components of
the date.