Vishnupurana 4.24.28

4.24.28

kauṭilya eva candraguptasutpannaṃ rājye 'bhiṣekṣyati //

rise in the heavens, and some lunar asterism is seen at night at an equal distance between them, then the seven Rishis continue stationary in that conjunction for a hundred years of men. At the birth of Paríkshit they were in Maghá, and the Kali age then commenced, which consists of 1200 (divine) years. When the portion of Vishńu (that had been born from Vasudeva) returned to heaven, then the Kali age commenced. As long as the earth was touched by his sacred feet, the Kali age could not affect it. As soon as the incarnation of the eternal Vishńu had departed, the son of Dharma, Yudhisht́hira, with his brethren, abdicated the sovereignty. Observing unpropitious portents, consequent upon Krishńa's disappearance, he placed Paríkshit upon the throne. When the seven Rishis are in Purváshád́há, then Nanda will begin to reign, and thenceforward the influence of the Kali will augment.