'Return formatted timezone offset (+xx:xx) or None.'

def _tzstr(self, sep=':'):

    off = self.utcoffset()
    if (off is not None):
        if (off.days < 0):
            sign = '-'
            off = (- off)
        else:
            sign = '+'
        (hh, mm) = divmod(off, timedelta(hours=1))
        assert (not (mm % timedelta(minutes=1))), 'whole    minute'
        mm //= timedelta(minutes=1)
        assert (0 <= hh < 24)
        off = ('%s%02d%s%02d' % (sign, hh, sep, mm))
    return off
