Bhagavad Gita 8.17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदु: |
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जना:

Translation

One day of Brahma (kalp) lasts a thousand cycles of the four ages (mahā yug) and his night also extends for the same span of time. The wise who know this understand the reality about day and night.