Bhagavad Gita, Chapter 8, Verse 17

सहस्रयुगपर्यन्तमहर्यद् ब्रह्मणो विदुः । रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः

sahasrayugaparyantamaharyad brahmaṇo viduḥ | rātriṃ yugasahasrāntāṃ te'horātravido janāḥ

Translation

They — those people who know day and night — know that the day of Brahmā is a thousand yugas long and the night is a thousand yugas long.

— Alladi Mahadeva Sastri

Word meanings

sahasrayugaparyantam
thousand, yugas, boundary, of which, that
ahaḥ
day
yat
which
brahmaṇaḥ
of Brahmā
viduḥ
know
rātrim
the night
yugasahasrāntām
of yugas, by (a) thousand, end, of which, that
te
they
ahorātravidaḥ
day, and, night, and, know, thus
janāḥ
people

Śaṅkarācārya's commentary

Brahmā is the Prajāpati, the Virāj. Those persons who know how to compute Time know that Brahmā's day is a thousand yugas long, and that His night is of the same duration as His day. Because the worlds are thus limited by time, therefore they return again.