Bhagavad Gita Chapter 09, Text 07

Bg 9.7

sarva-bhutani kaunteya
prakrtim yanti mamikam
kalpa-ksaye punas tani
kalpadau visrjamy aham

Word for word: 
sarva-bhutani — all created entities; kaunteya — O son of Kunti; prakrtim — nature; yanti — enter; mamikam — My; kalpa-ksaye — at the end of the millennium; punah — again; tani — all those; kalpa-adau — in the beginning of the millennium; visrjami — create; aham — I.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O son of Kunti, at the end of the millennium all material manifestations enter into My nature, and at the beginning of another millennium, by My potency, I create them again.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The creation, maintenance and annihilation of this material cosmic manifestation are completely dependent on the supreme will of the Personality of Godhead. “At the end of the millennium” means at the death of Brahma. Brahma lives for one hundred years, and his one day is calculated at 4,300,000,000 of our earthly years. His night is of the same duration. His month consists of thirty such days and nights, and his year of twelve months. After one hundred such years, when Brahma dies, the devastation or annihilation takes place; this means that the energy manifested by the Supreme Lord is again wound up in Himself. Then again, when there is a need to manifest the cosmic world, it is done by His will. Bahu syam: “Although I am one, I shall become many.” This is the Vedic aphorism (Chandogya Upanisad 6.2.3). He expands Himself in this material energy, and the whole cosmic manifestation again takes place.