ekāttara catur-yuge eka manv-antara
caudda manv-antara brahmāra divasa bhitara
ekāttara—seventy-one; catuḥ-yuge—in cycles of four ages; eka—one; manu-antara—period of a Manu; caudda—fourteen; manu-antara—periods of Manu; brahmāra—of Lord Brahmā; divasa—a day; bhitara—within.
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. 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 writing a commentary on the Sūrya-siddhānta, and the title Gosvāmī Mahārāja was added when he accepted sannyāsa, the renounced order of life.

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