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SB 3.11.35

pūrvasyādau parārdhasya
 brāhmo nāma mahān abhūt
kalpo yatrābhavad brahmā
 śabda-brahmeti yaṁ viduḥ
Synonyms: 
pūrvasya — of the first half; ādau — in the beginning; para-ardhasya — of the superior half; brāhmaḥ — Brāhma-kalpa; nāma — of the name; mahān — very great; abhūt — was manifest; kalpaḥ — millennium; yatra — whereupon; abhavat — appeared; brahmā — Lord Brahmā; śabda-brahma iti — the sounds of the Vedas; yam — which; viduḥ — they know.
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.