ahar yad brahmaṇo viduḥ
rātriḿ yuga-sahasrāntāḿ
te ’ho-rātra-vido janāḥ

Translation of Bhagavad Gita 8.17

By human calculation, a thousand ages taken together form the duration of Brahma’s one day. And such also is the duration of his night.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

The duration of the material universe is limited. It is manifested in cycles of kalpas. A kalpa is a day of Brahma, and one day of Brahma consists of a thousand cycles of four yugas, or ages: Satya, Treta, Dvapara and Kali. The cycle of Satya is characterized by virtue, wisdom and religion, there being practically no ignorance and vice, and the yuga lasts 1,728,000 years. In the Treta-yuga vice is introduced, and this yuga lasts 1,296,000 years. In the Dvapara-yuga there is an even greater decline in virtue and religion, vice increasing, and this yuga lasts 864,000 years. And finally in Kali-yuga (the yuga we have now been experiencing over the past 5,000 years) there is an abundance of strife, ignorance, irreligion and vice, true virtue being practically nonexistent, and this yuga lasts 432,000 years. In Kali-yuga vice increases to such a point that at the termination of the yuga the Supreme Lord Himself appears as the Kalki avatara, vanquishes the demons, saves His devotees, and commences another Satya-yuga. Then the process is set rolling again. These four yugas, rotating a thousand times, comprise one day of Brahma, and the same number comprise one night. Brahma lives one hundred of such “years” and then dies. These “hundred years” by earth calculations total to 311 trillion and 40 billion earth years. By these calculations the life of Brahma seems fantastic and interminable, but from the viewpoint of eternity it is as brief as a lightning flash. In the Causal Ocean there are innumerable Brahmas rising and disappearing like bubbles in the Atlantic. Brahma and his creation are all part of the material universe, and therefore they are in constant flux.

In the material universe not even Brahma is free from the process of birth, old age, disease and death. Brahma, however, is directly engaged in the service of the Supreme Lord in the management of this universe—therefore he at once attains liberation. Elevated sannyasis are promoted to Brahma’s particular planet, Brahmaloka, which is the highest planet in the material universe and which survives all the heavenly planets in the upper strata of the planetary system, but in due course Brahma and all the inhabitants of Brahmaloka are subject to death, according to the law of material nature.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

It is said in Bhagavatam:

amrtam ksemam abhayarh tri-murdhno ‘dhayi murdhasu

Deathlessness, fearlessness and freedom from the anxieties of old age and disease exist in the kingdom of God, which is beyond the three higher planetary systems and beyond the material coverings. SB 2.6.19

“But some say however that there is fearlessness on Brahmaloka as well, and thus, it is not possible for the sannyasi to die there, if they desire to stay there.”

No, if even Brahma, the master of that planet, must die, what can we say of others? Those who are knowledgeable of scripture, who know that the day of Brahma lasts a thousand yugas, also know that the night lasts a thousand yugas, because they are knowers of day and night. By such days, fortnights and months Pass. A hundred of such years make the life of Brahma. Then after that, Brahma dies. But this is not the case if Brahma is a Vaisnava. That Brahma will attain liberation.

Commentary by Sri Ramanuja of Sri Sampradaya:

8.17 These men who know the order of the day and night as established by My will in regard to all beings, beginning with man and ending with Brahma — they understand that what forms Brahma’s day is a unit comprising in it a thousand periods of four Yugas (Catur-yugas) and anight is a unit of equal duration.

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

In the Puranas the ancient Vedic histories there are statements like: The contemplative, the charitable, the dispassionate and the consequent reach a higher abode that is free from affliction above the material realms of the earthly worlds, the heavenly kingdom and the nether regions. It is known that above the three planetary systems Bhur, Bhuvah and Swah, the planetary system of Maharloka is far superior to the Earth; but if periodic destruction is anyway applicable to all material planetary systems then what is the special distinction. Anticipating such a question and in order to convey that existence on Maharloka lasts for a long duration compared to the much shorter duration of the three previously mentioned lokas or planetary systems being Bhur, Bhuvah and Swar below it. By showing that in each day of the life of Brahma which equal 4 billion, 320 million years and who lives one-hundred years according to this time scale, the material manifestation of the three worlds are created and on each night in the life of Brahma also equalling 4 billion 320 million years the three worlds are dissolved again. Both together total 8 billion 640 million.

Such is the proper method of computing time. One year of time on Earth equals one day and one night of the demigods. Computing further in gradation for them and Earth it can be tabulated that one Maha Yuga equals 4,320,000 Earth years but for the demigods that is only a durarion of 12,000 of their years. 1000 such Maha Yugas comprises Brahma’s day and another 1000 Maha Yugas comprises his night. Only those who understand time in this frame of reference are actual knowers of day and night. Those beings whose knowledge is restricted to the rotations of the sun and moon are not knowers of day and night for there knowledge is limited. The word yuga in this verse is referring to a Maha Yuga refering to the four yugas of Satya, Treta, Dvarapa and Kali equalling 4 million 320 thousand years. Otherwise combining it with sahasra-yuga would have no relevance to Brahma’s day and night. The Vishnu Purana states: Brahma’s day consists of 1000 Maha Yugas. The name Brahma also denotes the inhabitants of his planetary system of Maharloka as well.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

Lord Krishna in order to establish the fact that upon attaining Him there is no further return to the material existence, He cites the potency of His unmanifest Brahman by the example of the cyclic dissolution of creation referring to Brahma’s day. The literal meaning for sahasra is one 1000; but actually it means innumerable for in innumerable creations is dissolution taking place periodically. The Brahman or the spiritual substratum pervading all creation is a manifestation of the Supreme Lord and is supreme as well being the shadow of the universal form. Here Lord Krishna speaks of dissolution at the end of Brahma’s day. Everything comes into creation by the unmanifest. One Maha Yugas consists of Satya Yuga, Treta Yuga, Dvarapa Yuga and Kali Yuga equalling 4,320,000 years multiplied by 1000 is 4 billion 320 million years and is a day of Brahma. The same duration is his night both combined equal 8 billion 640 million years and which is known as a kalpa.. A year of Brahma is 360 of such kalpas totalling 26 trillion 438 billion and 400 million years and Brahma lives for 100 of these years. The Kurma Purana states: For as long as the life of Brahma is a day for the Supreme Lord and after begins His night. When His night falls everything gets merged back into Him and when His day dawns everything emanates out again and creation begins anew. The Supreme Being who resides in the etheric heart of all beings.

Now begins the summation.

The word sahasra means 1000 but it also indicative of the Brahman or the spiritual substratum pervading all existence. What is the source of the Brahman? It is emanating from the Supreme Being. Only the periodic cycles of night and day are spoken of in reference to the unmanifest Brahman. When night falls after 4 billion 320 million years everything gets dissolved back into the unmanifest Brahman. Thus from the statement can be drawn a conclusion. At the end of day and night of the four-faced Brahma all the universe is not dissolved, only portions. The Supreme Lord Krishna alone is eternal. He is transcendental to the constructs of time beyond both the night and the day. The night and day of Brahma’s lifetime are said to be like the closing and opening of His eyelids.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

Lord Krishna is now giving the reason why people who reach the heavenly worlds of Brahma and others are still subjected to samsara or the perpetual cycle of birth and death because of the periodic time factors these planets are regulated by. Those who by the truth of the Vedic scriptures know that Brahma’s day and night is comprised of 2000 Maha Yugas which is equivalent to the duration of the four yugas or ages being Satya Yuga consisting of 1,728,000 years, Treta Yuga consisting of 1,296,000 years, Dvarapa Yuga consisting of 864,000 years and Kali Yuga consisting of 432,000 years all totalling 4,320,000 years multiplied againg by 2000 factually know what is day and night because they know the true reality of time. All others who base their knowledge of day and night by the mere calculations of solar movements are not knowers of day and night and are limited in their consciousness. One year on Earth is equal to one day and one night in the heavenly spheres. One day and one night in the heavenly worlds multiplied by 12,000 years equals one Maha Yuga equalling 4,320, 000 years. 1000 Maha Yugas is one day of Brahma and another 1000 Maha Yuga is one night of Brahma both totalling 8 billion 640 million years and is known as a kalpa. One year of Brahma is 360 of these kalpass or 180,000 Maha Yugas which is 360 days multiplied by 8 billion 640 million which totals to 26 trillion, 438 billion and 400 million years on Earth. Brahma lives 100 of these years. One who understands this is a true knower of day and night.

So in this way all the material worlds are always temporary due to being periodically regulated by the time factor. Naturally beings who reach these worlds are subject to these periodic regulations and thus are subjected to rebirth in the worlds of mortals. This is the case in the highest material world of Brahma in Maharloka all the way down to the lower worlds such as Pataloka. It should be understood that in all material worlds there is rebirth.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

8.17 Viduh, they know; that ahah, day; brahmanah, of Brahma, of Prajapati, of Virat; yat, which; sahasra-yuga-paryantam, ends in a thousand yugas; and also the ratirm, night; yuga-sahasra-antam, which ends in a thousand yugas, having the same duration as the day. Who knows (these)? In reply the Lord says: Te, they; janah, poeple; ahoratra-vidah, who are the knowers of what day and night are, i.e. the people who know the measurement of time. Since the worlds are thus delimited by time, therefore they are subject to return. What happens during the day and the night of Prajapati is being stated:

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

8.17-19 Sahasra-etc., upto aharagama. Those who could see afar (great seers), see [actually] the night and day even in the case of Brahma as being marked [respectively] by the destruction and creation [of the world]. Accordingly, having risen from sleep, the same [Selves] continue their own respective activities every day; they theyselves, putting an end to their activities every night, remain exclusively in the form of Energy [of the Absolute]. In this manner they come to be again and again at the time of creation and of dissolution. No new, but only the self-same personal Souls are let loose. Their mutual difference in the form of the idea of the long and short lives is based only on the concept of time. This delimitation is unavoidable even in the case of the Prajapatis. Hence it is established that they too are cetainly of the nature of having evolution and dissolution. [The Lord] clarifies His [own] statement : ‘People do return from each and every world; but having attained Me, the Supreme Lord, they do not do so.’

Sanskrit Shloka Without Transliteration Marks:

ahar yad brahmano viduh
ratrim yuga-sahasrantam
te ’ho-ratra-vido janah

Sanskrit to English Word for Word Meanings:

sahasra — one thousand; yuga — millenniums; paryantam — including; ahaḥ — day; yat — that which; brahmaṇaḥ — of Brahmā; viduḥ — they know; rātrim — night; yuga — millenniums; sahasra-antām — similarly, ending after one thousand; te — they; ahaḥ-rātra — day and night; vidaḥ — who understand; janāḥ — people.