Comvert datetime to short text.
    If datetime has timezone then it will be convert to UTC0.

def datetime_to_dtstr(dt=None):
    """
    Comvert datetime to short text.
    If datetime has timezone then it will be convert to UTC0.
    """
    if dt is None:
        dt = datetime.datetime.utcnow()
    elif timezone.is_aware(dt):
        dt = dt.astimezone(tz=pytz.UTC)
    return int2base36(int(time.mktime(dt.timetuple()) * 1e3 + dt.microsecond / 1e3))