The creation, maintenance and annihilation of this material cosmic manifestation is completely dependant 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 need to manifest the cosmic world, it is done by His will: "Although I am one, I shall become many." This is the Vedic aphorism. He expands Himself in this material energy, and the whole cosmic manifestation again takes place.