8.17 Viduh, they know; that ahah, day; brahmanah, of Brahma, of Prajapati, of Virat; yat, which; sahasra-yuga-paryantam, ends in a thousand yugas; and also the ratirm, night; yuga-sahasra-antam, which ends in a thousand yugas, having the same duration as the day. Who knows (these)? In reply the Lord says: Te, they; janah, poeple; ahoratra-vidah, who are the knowers of what day and night are, i.e. the people who know the measurement of time. Since the worlds are thus delimited by time, therefore they are subject to return.
What happens during the day and the night of Prajapati is being stated: