17. Those who know the day of Brahma to be of thousands of ages and the nights to be of thousands of ages, they are, verily, the knowers of the day and night.
Bhashya :- To establish the fact that after attaining the supreme Lord there is no return to the primordial world, his power as the unmanifest is shown through the instances of dissolution (of universe) etc. सहस` means countless. Brahman is the Supreme One. “He is the night of the universal form”, thus speak the scriptures. Here He speaks of the dissolution at the end of the cycle. “Everything comes to be created by the un-manifest”.
Tatparya Nirnaya :- सह्स` means immeasurable. ब्रह्मण: is supremely immeasurable, infinite. The unmanifest who has become manifest, only the nights and days being spoken in reference to unmanifest Brahman. The arrival of the night is when the universe gets dissolved by the four-faced Brahma. Sri Hari alone is the Eternal Time, He himself being both night and the day. The night and day is said to be like the closing and opening his eye-lids, as formal statement.