B.G 8.17, 18, 19 and 20
Those who know the nature of day and night understand that the day of Brahmā lasts for a thousand yugas, and his night also ends after a thousand yugas.
From the unmanifest, all manifest beings arise at the beginning of the day (of Brahmā), and at the arrival of the night, they dissolve back into that which is called the unmanifest.
O Pārtha (Arjuna), this multitude of beings, being born again and again, dissolves at the arrival of the night (of Brahmā) helplessly, and at the arrival of the day, it comes into existence again.
But beyond that unmanifest (existence), there is another eternal unmanifest reality, which does not perish even when all beings perish.
Gīta Tātparya 8.17, 18, 19 and 20
The Supreme Brahman is not destroyed when the dissolution of all manifested beings occurs. 'sahasra' - many or infinite. 'brahmaṇa' - the Supreme Brahman.
"Sahasram" (a thousand) signifies multitude or infinite. It word 'brahmaṇa' refers to the Supreme Brahman.
"All manifest beings arise from the unmanifest at the arrival of the day. At the arrival of the night, they dissolve into that very unmanifest, which is so designated."
- stated thus in the remainder of the statement.
Indeed, the Brahman is not destroyed when the dissolution of all manifested beings occurs.
"Even though Lord Hari is eternal, His time is considered in terms of two parārddhas each for a day and night. It is indeed a mere blink of an eye for the inactive One."
- stated thus as well.
