Chapter 8 · The Imperishable Absolute · Krishna said
Bhagavad Gita 8.17 — Brahma's day and night
सहस्रयुगपर्यन्तमहर्यद् ब्रह्मणो विदुः। रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ॥
sahasrayugaparyantamaharyad brahmaṇo viduḥ | rātriṃ yugasahasrāntāṃ te'horātravido janāḥ ||
Meaning
Those who know that a day of Brahma lasts a thousand ages, and his night too lasts a thousand ages, truly know what day and night are.