Sanskrit
सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः । रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ॥
sahasrayugaparyantam ahar yad brahmaṇo viduḥ rātriṃ yugasahasrāntāṃ te'horātravido janāḥ
Translation
The people who know the day of Brahmâ, a thousand ages in duration, and the night a thousand ages in ending, they know day and night.
Annie Besant · The Bhagavad-Gita, translated by Annie Besant (4th ed., 1922) — Wikisource · Public Domain
Explanation
Having just invoked “the world of Brahma” as something that still returns, Krishna now supplies its timescale: a single day of Brahma equals a thousand yugas, and the night another thousand. The number matters less than the function — it recalibrates human notions of permanence, making even Brahma’s vast lifespan look small next to the higher, genuinely eternal reality verses 20-21 are about to introduce.