Should this update existing 'Event' instances to set updated_at?  Otherwise the /api/events service returns an error for me:

    ...
      File "/vagrant/src/ggrc/services/common.py", line 467, in http_timestamp
        return format_date_time(time.mktime(timestamp.utctimetuple()))
    AttributeError: 'NoneType' object has no attribute 'utctimetuple'