'Input value validation, typically overridden by derived classes'

def _check_val_type(self, val1, val2):

    if (hasattr(val1, u'to') and hasattr(val1, u'unit')):
        raise ValueError(u"Cannot    use    Quantities    for    'byear'    format,    as    the    interpretation    would    be    ambiguous.    Use    float    with    Besselian    year    instead.    ")
    return super(TimeBesselianEpoch, self)._check_val_type(val1, val2)
