One verse

Bhagavad Gita 8.17

8.17

सहस्रयुगपर्यन्तमहर्यद् ब्रह्मणो विदुः | रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ||८-१७||

sahasrayugaparyantamaharyad brahmaṇo viduḥ . rātriṃ yugasahasrāntāṃ te.ahorātravido janāḥ ||8-17||

sahasra
one thousand
yuga
millenniums
paryantam
including
ahaḥ
day
yat
that which
brahmaṇaḥ
of Brahmā
viduḥ
they know
rātrim
night
yuga
millenniums
sahasra-antām
similarly, ending after one thousand
te
they
ahaḥ-rātra
day and night
vidaḥ
who understand
janāḥ
people

By human calculation, a thousand ages taken together form the duration of Brahmā’s one day. And such also is the duration of his night.