Bhagavad Gita: Chapter 8, Verse 17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदु: |
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जना: || 17||

sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ
rātriṁ yuga-sahasrāntāṁ te ’ho-rātra-vido janāḥ

sahasraone thousand; yugaage; paryantamuntil; ahaḥone day; yatwhich; brahmaṇaḥof Brahma; viduḥknow; rātrimnight; yuga-sahasra-antāmlasts one thousand yugas; tethey; ahaḥ-rātra-vidaḥthose who know his day and night; janāḥpeople

Translation

BG 8.17: One day of Brahma (kalp) lasts a thousand cycles of the four ages (mahā yug) and his night also extends for the same span of time. The wise who know this understand the reality about day and night.

Commentary

The measurements of time in the Vedic cosmological system are also vast and staggering. For example, there are insects that are born in the night; they grow up, procreate, lay eggs, and grow old, all in one night. In the morning, you see then all dead under the street lights. If these insects were told that their entire lifespan was only one night of human beings, they would find it incredulous.

Similarly, the Vedas state that one day and night of the celestial gods, such as Indra and Varun, corresponds to one year on the earth plane. One year of the celestial gods, consisting of 30 × 12 days is equal to 360 years on the earth plane. 12,000 years of the celestial gods correspond to one mahā yug (cycle of four yugas) on the earth plane, i.e. 4 million and 320 thousand years. This is the highest unit of time in the world, and is called a kalp.

1,000 such mahā yugas comprise one day of Brahma,. This is called kalp and is the largest unit of time in the world. Equal to that is Brahma’s night. By these calculations, Brahma lives for 100 years. By earth calculations, it is 311 trillion 40 billion years.

Thus, the Vedic calculations of time are as follows:

Kali Yug: 432,000 years.

Dwāpar Yug: 864,000 years.

Tretā Yug: 1,296,000 years.

Satya Yug: 1,728,000 years.

Together, they comprise a Mahā Yug: 4,320,000 years.

One thousand Mahā Yugas comprise one day of Brahma, which is a kalp: 43,200,000 years. Of equal duration is Brahma’s night. Shree Krishna says that those who understand this are the true knowers of day and night.

The entire duration of the universe is equal to Brahma’s lifespan of 100 years: 311 trillion 40 billion years. Brahma is also a soul, who has attained that position and is discharging his duties for God. Hence, Brahma is also within the cycle of life and death. However, being of extremely elevated consciousness, he is assured that at the end of his life, he will be released from the cycle of life and death and go to the abode of God. Occasionally, when no soul is eligible to perform the duties of Brahma at the time of the creation of the world, God himself becomes Brahma.