'Validate that value in "HH:MM:SS" format and convert to timedelta.
Validate that user, that edits XML, sets proper format, and
that max value that can be used by user is "23:59:59".'

@classmethod
def isotime_to_timedelta(cls, value):

    try:
        obj_time = time.strptime(value, '%H:%M:%S')
    except ValueError as e:
        raise ValueError('Incorrect    RelativeTime    value    {!r}    was    set    in    XML    or    serialized.    Original    parse    message    is    {}'.format(value, e.message))
    return datetime.timedelta(hours=obj_time.tm_hour, minutes=obj_time.tm_min, seconds=obj_time.tm_sec)
