सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः। रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ॥

sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ rātriṁ yuga-sahasrāntāṁ te ’ho-rātra-vido janāḥ

The people who know the day of Brahmâ, a thousand ages in duration, and the night a thousand ages in ending, they know day and night.

In plain words: Krishna explains cosmic time. One day of Brahma, the creator, lasts a thousand great ages, and his night lasts just as long. People who understand this truly know what day and night mean.