get timezone as tz_offset

def convert_timezone(date_str, tz_from, tz_to="UTC", fmt=None):
    """ get timezone as tz_offset """
    tz_offset = datetime_to_timezone(
        datetime.datetime.now(), tz=tz_from).strftime('%z')
    tz_offset = tz_offset[:3] + ':' + tz_offset[3:]

    date = parse_date(str(date_str) + tz_offset)
    if tz_from != tz_to:
        date = datetime_to_timezone(date, tz_to)

    if isinstance(fmt, str):
        return date.strftime(fmt)
    return date