Bhagavad Gita 8.17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः।रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः॥

sahasrayugaparyantamaharyadbrahmaṇo viduḥ |rātriṃ yugasahasrāntāṃ te'horātravido janāḥ ||

Those who know the day of Brahma, which lasts a thousand Yugas, and the night, which also lasts a thousand Yugas, know day and night.

Those who understand the cosmic day and night know that one day of creation is a thousand cycles, and that the night is of equal length.

Our plain-language meaning of this verse is waiting for scholar review.