In plain words
Those who know day and night understand Brahma’s day to last a thousand yugas, and Brahma’s night also to last a thousand yugas.
Word for word
Those people who are knowers of day and night know the day of Brahmā which ends with a thousand yugas, [and his] night ending with a thousand yugas.
सहस्रयुगपर्यन्तमहर्यद् ब्रह्मणो विदुःरात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
ब्रह्मलोकसहिताः लोकाः कस्मात् पुनरावर्तिनः ? कालपरिच्छिन्नत्वात् । कथम् ? —
सहस्रयुगपर्यन्तं सहस्राणि युगानि पर्यन्तः पर्यवसानं यस्य अह्नः तत् अहः सहस्रयुगपर्यन्तम् , ब्रह्मणः प्रजापतेः विराजः विदुः, रात्रिम् अपि युगसहस्रान्तां अहःपरिमाणामेव । के विदुरित्याह — ते अहोरात्रविदः कालसङ्ख्याविदो जनाः इत्यर्थः । यतः एवं कालपरिच्छिन्नाः ते, अतः पुनरावर्तिनो लोकाः ॥ १७ ॥
In plain words
Even the world of Brahmā, and every world like it, still brings you back — why? Because every one of them is bound by time. And how exactly? —
'Extending to a thousand yugas' — a single day of Brahmā, of Prajāpati, of the Virāj, lasts exactly that long: a thousand yugas from start to finish. And the night is just the same length as the day. Who understands this? Those who are called 'knowers of day and night' — meaning, simply, people who understand how vast cosmic time actually is. And because even Brahmā's days and nights are measured out and bounded like this, the worlds one reaches through such practice still bring you back.
Close to the Sanskrit
The worlds, including the world of Brahmā — why are they subject to return? Because they are limited by time. How? —
'Extending to a thousand yugas': a thousand yugas is the extent, the termination, of which day — that day is 'extending to a thousand yugas' — they know this of Brahmā, of Prajāpati, of the Virāj; and the night too, ending in a thousand yugas, of exactly the same measure as the day. Who knows this? — he says: they are the 'knowers of day and night,' that is, people who know the reckoning of time — this is the meaning. Since these are thus limited by time, therefore the worlds are subject to return.