'Convert a time tuple from Gregorian calendar to this calendar
@param timetuple: time tuple (y, m, d, hh, mm, ss)
@return: time tuple (this calendar)'

def _cdate(self, timetuple):

    if (self.name == 'Gregorian'):
        return timetuple
    (y, m, d, hh, mm, ss) = timetuple
    jd = self._gregorian_to_jd(y, m, d)
    (y, m, d) = self.from_jd(jd)
    return (y, m, d, hh, mm, ss)
