Bhagavad Gita Chapter 8 Verse 17
भगवद् गीता अध्याय 8 श्लोक 17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः।
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः।।8.17।।



English Translation - Swami Sivananda
8.17 Those people who know the day of Brahma which is of a duration of a thousand Yugas (ages) and the night which is also of a thousand Yugas duration, they know day and night.


English Translation of Sanskrit Commentary By Sri Shankaracharya's
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:


Transliteration Bhagavad Gita 8.17Sahasrayugaparyantam aharyad brahmano viduh;
Raatrim yugasahasraantaam te’horaatravido janaah.


Word Meanings Bhagavad Gita 8.17sahasra—one thousand; yuga—age; paryantam—until; ahaḥ—one day; yat—which; brahmaṇaḥ—of Brahma; viduḥ—know; rātrim—night; yuga-sahasra-antām—lasts one thousand yugas; te—they; ahaḥ-rātra-vidaḥ—those who know his day and night; janāḥ—people