Bhagavad Gita 8.17सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः।रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ॥

Day means evolution or projection or manifestation of the universe. Night means involution of the universe or Pralaya. The worlds are limited or conditioned in time. Therefore they return again. The world of Brahma (Brahmaloka or Sattyaloka) is also transient, although it lasts for a thousand ages. When the four great Yugas have gone round a thousan times, it make a daytime of Brahma and when an eal number of Yugas pass again it makes a night. Those who can see and live through the day and night of Brahma can really know what is a day and what is a night.The Suryasiddhanta speaks of the same division of time.According to it YearsKaliyuga (with its Sandhya andSandhyamsa) consists of 432,000Dvapara Yuga (do) 864,000Tretayuga (do) 1,296,000Kritayuga (do) 1,728,000Thus a Mahayuga consisting ofthese four Yugas comprises 4,320,00071 such Mahayugas with an additionalSandhya, at the close of 1,728,000years make one Manvantara of 308,448,00014 such Manvantaras with anotherSandhya, at the close of 1,728,000years constitute one Kalpa of 4,320,000,000Two Kalpas make a day and nightof Brahma of 8,640,000,000360 such days and nights make oneyear of Brahma consisting of 3,110,400,000,000100 such years constituteHis lifetime of 311,040,000,000,000The world is absorbed in the Avyakta or the Unmanifested or Mulaprakriti during the cosmic Pralaya (involution of the world)