The day whose end, whose limit, is a thousand ages, that day is a thousand ages long; this is the day of Brahmā, of Prajāpati, of Virāj, that those who know declare; and the night too is the end of a thousand ages, of the same measure as the day. Who are they that know? They are the people who know day and night, who know the reckoning of time. Since the worlds are thus bounded by time, they are therefore subject to return. What happens in Prajāpati's day and in his night is told.

This is a brief sub-gloss. For a fuller reading of this verse, see Madhusūdana, Śaṅkara, or Rāmānuja above.

Those people who know the ordering of day and night, wrought by My resolve, of beings from man up to the four-faced one, they know that the day of Brahma, the four-faced one, ends with a thousand cycles of four ages, and the night likewise.

To establish that, on reaching Me, there is no return, and to show the power of the Self named the unmanifest, Krishna shows the dissolution and the rest, with 'a thousand ages' and so on. The word 'thousand' here means 'many'. It concerns Brahma; the scripture says, 'that is the night of the one of universal form'. What is meant here is the dissolution at the end of the two halves of Brahma's life, since it is said 'from the unmanifest all the manifestations arise' (8.18). And it is said in the Maha-kaurma, 'the night of the great Vishnu likewise lasts to the end of many ages; at the beginning of the night all dissolves, and at the beginning of the day it is born'. This follows too from the closing words, 'that which, in all beings' (8.20).