Mudgalapurana 3.15.33

3.15.33

dakṣiṇāyanarūpā vai rātristeṣāṃ prakīrtitā |
evaṃ triṃśadbhirākhyāto māsaścaiva mahāmune ||

The night of theirs is proclaimed to consist of the southern transit. / In this way, by thirty [such days and nights], a month is described, O great sage;