8:17
सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः ।
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ॥८- १७॥
sahasrayugaparyantamaharyadbrahmaNo widuH ।
raatriM yugasahasraantaaM te'horaatrawido janaaH ॥८- १७॥
Source: Automatic ITRANS Transliteration
Who know that Brahma's day spans a thousand yugas,
Night ends in a thousand yugas, [those] people know day and night.
Source: Milind S. Pandit
Those who know that the day of Brahmaa lasts one thousand Yugas (or 4.32 billion years) and that his night also lasts one thousand Yugas, they are the knowers of day and night.
By human calculation, a thousand ages taken together is the duration of Brahma's one day. And such also is the duration of his night.
Source: Bhagavad Gita As It Is