Normalize a time. If the time is tz-naive, assume it is UTC.

def ensure_utc(time, tz='UTC'):
    """
    Normalize a time. If the time is tz-naive, assume it is UTC.
    """
    if not time.tzinfo:
        time = time.replace(tzinfo=pytz.timezone(tz))
    return time.replace(tzinfo=pytz.utc)