Those people who know the day of Brahma which is of a duration of a thousand Yugas (ages) and the night which is also of a thousand Yugas duration, they know day and night.
In simple words
Krishna describes cosmic timescales: "Those who understand the vastness of cosmic time know this — one 'day' of the Creator lasts a thousand ages, and one 'night' lasts another thousand ages."
Word-by-word meanings
सहस्रयुगपर्यन्तम्ending in a thousand Yugas (ages)अहःdayयत्whichब्रह्मणःof Brahmaविदुःknowरात्रिम्the nightयुगसहस्रान्ताम्ending in a thousand Yugas (ages)तेtheyअहोरात्रविदःknowers of day and nightजनाःpeople
8.17 सहस्रयुगपर्यन्तम् ending in a thousand Yugas (ages)? अहः day? यत् which? ब्रह्मणः of Brahma? विदुः know? रात्रिम् the night? युगसहस्रान्ताम् ending in a thousand Yugas (ages)? ते they? अहोरात्रविदः knowers of day and night? जनाः people.Commentary 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). Just as the tree remains in a latent state in the seed? so also this whole universe remains in a latent state in a seedform in the Mulaprakriti during Pralaya. This is the night of Brahma. This is the cosmic night. Again the world is projected at the beginning of the Mahakalpa (evolution). There comes the cosmic dawn or cosmic day. This eternal rhythm of cosmic day and night (evolution and involution) is kept up in the macrocosm.Nothing that comes under this everrevolving wheel of cosmic day and night lasts for ever. That is the reason why the seers of the Upanishads? the sages of yore? lived in the transcendental Supreme being? the imperishable Self? the indestructible Purusha? the supreme goal of life? the highest end of man? which is beyond the cosmic day and night. Just as the seeds that are fried can hardly germinate? so also those who have attained to the imperishable Brahman? the Absolute? the Eternal? cannot return to this world of sorrow? pain and misery. They know neither day nor night. They are one with Existence Absolute.The manifested and the unmanifested dwell in Brahman. Brahman is beyond the manifested and the unmanifested. When the world and the body are destroyed Brahman is not destroyed. The waves come out and subside? but the ocean remains unaffected. So also the worlds come and subside? but Brahman the source of everything? the source of Mulaprakriti? ever remains unaffected. Just as ornaments come out of gold and then go back to gold when they are melted? so also all the worlds come out of Brahman and go back to Brahman. Gold is in no way affected by the various forms such as earning? bracelets? anklets? etc.? that have been made of it. Even so Brahman is not in the least affected by the projection and destruction (dissolution) of the worlds and the bodies of beings. It remains always as It is.
Contemporary scholarly and practical interpretations for modern seekers.
This interpretation draws on a specific tradition and may not represent the view of any single school. For authoritative guidance within a specific tradition, seek a qualified teacher.
Modern
Einstein's theory of relativity has unveiled a mystery that is now accepted in Western countries. According to this principle, our conceptions of space and time depend upon individual factors that regulate their measurement. When the mind is disturbed, time feels heavy and seems to pass slowly—as when a person waits in anxiety or intense eagerness. Yet that same person experiences time as flying when resting in pleasant circumstances where the mind finds enjoyment. A man absorbed in playing cards does not notice how the night has passed, and is astonished to see the sun's rays entering through the window at dawn. When one must act against the mind's inclination or when the body suffers pain, each moment feels like ages. In the unbroken experience of sleep, there is no perception of time at all.
From observing these phenomena, Hindu philosophers arrived at this rational conclusion: what we truly call time is the measurement of the interval between two different experiences. The greater the number of experiences that disturb the mind, the slower time seems to pass. If a single experience persists for a long duration, time passes swiftly. In a single experience alone, time is not perceived—just as distance cannot be measured at a single point, for distance is measured only by calculating the interval between two points. Based on this principle, when the ancient poets said that the dials of the gods' clocks are larger, their statement was indeed apt. The Upanishads too have examined bliss, wherein taking one unit of human joy as the measure, they calculated the measure of bliss enjoyed by the gods up to Brahma. The increase in the measure of bliss in the various higher worlds above the mortal realm reflects the correspondence between the peace and equanimity of mind attained in those worlds.
In this verse, it is said that one day of Brahma comprises a thousand yugas, and one night is equally long. By yuga is meant a kalpa. The calculation made by those versed in time is according to our reckoning of a year of 365 days. Four yugas make one kalpa, and one day of Brahma is reckoned as a thousand kalpas.
As the individual units are, so shall the collective be. The individual mind creates the universe of its own accord and sustains it as well. Thereafter it destroys it—only to create a new universe again. This continuous work of creation and dissolution, the human being performs only during the day, that is, in the waking state alone. Similarly, it is understood that the collective mind—Brahma—creates this entire universe of the moving and the stationary only during his waking state.