SB 3.11.35
purvasyadau parardhasya
brahmo nama mahan abhut
kalpo yatrabhavad brahma
sabda-brahmeti yam viduh
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In the beginning of the first half of Brahma’s life, there was a millennium called Brahma-kalpa, wherein Lord Brahma appeared. The birth of the Vedas was simultaneous with Brahma’s birth.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
According to Padma Purana (Prabhasa-khanda), in thirty days of Brahma many kalpas take place, such as the Varaha-kalpa and Pitr-kalpa. Thirty days make one month of Brahma, beginning from the full moon to the disappearance of the moon. Twelve such months complete one year, and fifty years complete one parardha, or one half the duration of the life of Brahma. The Sveta-varaha appearance of the Lord is the first birthday of Brahma. The birth date of Brahma is in the month of March, according to Hindu astronomical calculation. This statement is reproduced from the explanation of Srila Visvanatha Cakravarti Thakura.