akṣarabrahmayoga - Verse śloka 17
śloka 17
sahasrayugaparyantamaharyad brahmaṇo viduḥ |
rātriṃ yugasahasrāntāṃ te'horātravido janāḥ ||8-17||
Translation
Those who know the duration of Brahma's one day, which is a thousand years, and his one night of a thousand years—they are knowers of day and night.