'Return a validated scale value.
If there is a class attribute \'scale\' then that defines the default /
required time scale for this format.  In this case if a scale value was
provided that needs to match the class default, otherwise return
the class default.
Otherwise just make sure that scale is in the allowed list of
scales.  Provide a different error message if `None` (no value) was
supplied.'

def _check_scale(self, scale):

    if (hasattr(self.__class__, u'epoch_scale') and (scale is None)):
        scale = self.__class__.epoch_scale
    if (scale is None):
        scale = u'utc'
    if (scale not in TIME_SCALES):
        raise ScaleValueError(u"Scale    value    '{0}'    not    in    allowed    values    {1}".format(scale, TIME_SCALES))
    return scale
