'If other is of same class as self, return difference in self.scale.
Otherwise, raise OperandTypeError.'

def _time_difference(self, other, op=None):

    if (other.__class__ is not self.__class__):
        try:
            other = self.__class__(other, scale=self.scale)
        except Exception:
            raise OperandTypeError(self, other, op)
    if (((self.scale is not None) and (self.scale not in other.SCALES)) or ((other.scale is not None) and (other.scale not in self.SCALES))):
        raise TypeError(u"Cannot    compare    TimeDelta    instances    with    scales    '{0}'    and    '{1}'".format(self.scale, other.scale))
    if ((self.scale is not None) and (other.scale is not None)):
        other = getattr(other, self.scale)
    return ((self.jd1 - other.jd1) + (self.jd2 - other.jd2))
