Vishnupurana 1.3.12

1.3.12

catvāri trīṇi dve caikaṃ kṛtādiṣu yathākramam
divyābdānāṃ sahasrāṇi yugeṣv āhuḥ purāvidaḥ //

Oh best of sages, fifteen twinklings of the eye make a Kásht́há; thirty Kásht́hás, one Kalá; and thirty Kalás, one Muhúrtta. Thirty Muhúrttas constitute a day and night of mortals: thirty such days make a month, divided into two half-months: six months form an Ayana (the period of the sun's progress north or south of the ecliptic): and two Ayanas compose a year. The southern Ayana is a night, and the northern a day of the gods. Twelve thousand divine years, each composed of (three hundred and sixty) such days, constitute the period of the four Yugas, or ages. They are thus distributed: the Krita age has four thousand divine years; the Tretá three thousand; the Dwápara two thousand; and the Kali age one thousand: so those acquainted with antiquity have declared. The period that precedes a Yuga is called a Sandhyá, and it is of as many hundred years as there are thousands in the Yuga: and the period that follows a Yuga, termed the Sandhyánsa, is of similar duration. The interval between the Sandhyá and the Sandhyánsa is the Yuga, denominated Krita, Tretá, &c. The Krita, Tretá, Dwápara, and Kali, constitute a great age, or aggregate of four ages: a thousand such aggregates are a day of Brahmá, and fourteen Menus reign within that term. Hear the division of time which they measure.