Surya Siddhanta
सावहाहानि चान्द्रेभ्यो द्युभ्यः प्रोज्झ्य तिथिक्षयाः। उदयाद् उदयम् भानोर् भूमिसावनवासरः ॥
If the Sávana (terrestrial) days be subtracted from the Lunar days, the remainder constitute the days called the Tithi-kshaya (subtractive days.) There the Sávana days are those in which a Sávana day or terrestrial[2] day is equal to the time from sun-rise to sun-rise (at the equator).
english translation
sAvahAhAni cAndrebhyo dyubhyaH projjhya tithikSayAH। udayAd udayam bhAnor bhUmisAvanavAsaraH ॥
hk transliteration by Sanscript