Chapter 8 · verse 17 · gita.yoga.org

Bhagavad Gita 8.17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः। रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः

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.