The arithmetic of the mahayuga is a machine for converting myth into number. The Puranas and the Manusmriti begin with a divine year: one year of the gods equals 360 human years. The four ages are then measured in divine years, with each age bordered by a dawn and a twilight worth a tenth of its length apiece. The Krita or Satya age runs 4,000 divine years with junctions of 400 each, so 4,800 in all; the Treta 3,600; the Dvapara 2,400; the Kali 1,200. The declining sequence 4, 3, 2, 1 is the whole moral cosmology in miniature: dharma stands on four feet in the first age and loses one foot per age, virtue, lifespan and stature shrinking together.
The total, 12,000 divine years, multiplied by 360, yields 4,320,000 human years for one complete cycle. The figure's elegance is not accidental. Its factors, 432, 108, 60, tie it to the sexagesimal habits of ancient astronomy and to the sacred number 108 that pervades Indian tradition, and multiplying onward gives the larger scaffolding: a thousand mahayugas to the day of Brahma. Whatever one makes of the numbers, their scale did real work: they accustomed an entire civilization to thinking of time as vast beyond human measure, in which whole world-ages are single beats of a longer rhythm.
Sources Vishnu PuranaAnonymous with Manusmriti (The Laws of Manu)Anonymous