BG 8.17
सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः
sahasrayugaparyantamaharyadbrahmaṇo viduḥ
rātriṃ yugasahasrāntāṃ te'horātravido janāḥ
TRANSLATION
Those who know the duration of the day and night understand a day of Brahma to last for a thousand Yugas, and a night of Brahma to last for another thousand Yugas.
PURPORT
These men who know the order of the day and night as established by My will in regard to all beings, beginning with man and ending with Brahma—they understand that what forms Brahma’s day is a unit comprising in it a thousand periods of four Yugas (Catur-yugas) and anight is a unit of equal duration.