CC Ādi 3.8
চৌদ্দ মন্বন্তর ব্রহ্মার দিবস ভিতর ॥ ৮ ॥
caudda manv-antara brahmāra divasa bhitara
Seventy-one divya-yugas constitute one manv-antara. There are fourteen manv-antaras in one day of Brahmā.
A manv-antara is the period controlled by one Manu. The reign of fourteen Manus equals the length of one day (twelve hours) in the life of Brahmā, and the night of Brahmā is of the same duration. These calculations are given in the authentic astronomy book known as the Sūrya-siddhānta. An annotated Bengali translation of this book was compiled by the great professor of astronomy and mathematics Bimal Prasād Datta, later known as Bhaktisiddhānta Sarasvatī Gosvāmī, who was our merciful spiritual master. He was honored with the title Siddhānta Sarasvatī for translating the Sūrya-siddhānta, and the title Gosvāmī Mahārāja was added when he accepted sannyāsa, the renounced order of life.