Manasara 9.37

9.37

navabhirguṇite triṃśat kṣapeccheṣaṃ tithirbhavet
evamāyādiṣaḍvargaṃ kuryāttatra vicakṣaṇaiḥ

Multiplied by nine, divided by thirty, the remainder will be the lunar day. / Thus the group of six beginning with āya should be calculated by the wise.