Surya Siddhanta
नाडीषष्ट्या तु नाक्षत्रम् अहोरात्रम् प्रकीर्तितम्। तत्त्रिम्शता भवेन् मासः सावनो +अर्कोदयैस् तथा ॥
The time, which contains sixty Ghaṭikás is called a Nákshatra Ahorátra (a sidereal day and night) and a Nákshatra Mása (a sidereal month) consists of thirty Nákshatra Ahorátras. Thirty Sávana (terrestrial) days (a terrestrial day being reckoned from sun-rise to sun-rise) make a Sávana month.
english translation
nADISaSTyA tu nAkSatram ahorAtram prakIrtitam। tattrimzatA bhaven mAsaH sAvano +arkodayais tathA ॥
hk transliteration by Sanscript