Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 35

Sanskrit:

पूर्वस्यादौ परार्धस्य ब्राह्मो नाम महानभूत् ।कल्पो यत्राभवद्ब्रह्मा शब्दब्रह्मेति यं विदु: ॥ ३५ ॥

ITRANS:

pūrvasyādau parārdhasyabrāhmo nāma mahān abhūtkalpo yatrābhavad brahmāśabda-brahmeti yaṁ viduḥ

Translation:

In the beginning of the first half of Brahmā’s life, there was a millennium called Brāhma-kalpa, wherein Lord Brahmā appeared. The birth of the Vedas was simultaneous with Brahmā’s birth.

Purport:

According to Padma Purāṇa (Prabhāsa-khaṇḍa), in thirty days of Brahmā many kalpas take place, such as the Varāha-kalpa and Pitṛ-kalpa. Thirty days make one month of Brahmā, beginning from the full moon to the disappearance of the moon. Twelve such months complete one year, and fifty years complete one parārdha, or one half the duration of the life of Brahmā. The Śveta-varāha appearance of the Lord is the first birthday of Brahmā. The birth date of Brahmā is in the month of March, according to Hindu astronomical calculation. This statement is reproduced from the explanation of Śrīla Viśvanātha Cakravartī Ṭhākura.