Return a date string for an epoch-millis timestamp.

    Args:
        tick (int): The timestamp in milliseconds since the epoch.

    Returns:
        (str):  A date time string

def repr(tick, pack=False):
    '''
    Return a date string for an epoch-millis timestamp.

    Args:
        tick (int): The timestamp in milliseconds since the epoch.

    Returns:
        (str):  A date time string
    '''
    if tick == 0x7fffffffffffffff:
        return '?'

    dt = datetime.datetime(1970, 1, 1) + datetime.timedelta(milliseconds=tick)
    millis = dt.microsecond / 1000
    if pack:
        return '%d%.2d%.2d%.2d%.2d%.2d%.3d' % (dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second, millis)
    return '%d/%.2d/%.2d %.2d:%.2d:%.2d.%.3d' % (dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second, millis)