Skandapurana 16.105.52

16.105.52

pitṛkalpastathāṃte ca yā kuhūrbrahmaṇaḥ smṛtā ..
triṃśatkalpāḥ samākhyātā brahmaṇo māsi vai priye .. 52 ..

And at the end is the Pitṛkalpa, which is regarded as the new-moon day of Brahmā. / Thus thirty kalpas are enumerated in a month of Brahmā, O beloved!