Mbh 12.231.5a

"'"Vyasa said, 'Only Brahma, which is without beginning and without end, unborn, blazing with effulgence, above decay, immutable, indestructible, inconceivable, and transcending knowledge, exists before the Creation.[872] The Rishis, measuring time, have named particular portions by particular names. Five and ten winks of the eye make what is called a Kashtha. Thirty Kashthas would make what is called a Kala. Thirty Kalas, with the tenth part of a Kala added, make what is known as a Muhurta. Thirty Muhurtas make up one day and night. Thirty days and nights are called a month, and twelve months are called a year. Persons conversant with mathematical science say that a year is made up of two ayanas (dependent on sun's motion), viz., the northern and the southern. The sun makes the day and the night for the world of man. The night is for the sleep of all living creatures, and the day is for the doing of action. A month of human beings is equal to a day and night of the Pitris. That division (as regards the Pitris) consists in this: the lighted fortnight (of men) is their day which is for the doing of acts; and the dark fortnight is their night for sleep. A year (of human beings) is equal to a day and night of the gods. The division (as regards the gods) consists in this: the half year for which the sun travels from the vernal to the autumnal equinox is the day of the deities, and the half year for which the sun travels from the latter to the former is their night. Computing by the days and nights of human beings about which I have told thee, I shall speak of the day and night of Brahman and his years also. I shall, in their order, tell thee the number of years, that are (thus) for different purposes computed differently in respect of the Krita, the Treta, the Dwapara, and the Kali yugas. Four thousand years (of the deities) is the duration of the first or Krita age. The morning of that epoch consists of four hundred years and its evening is of four hundred years.