YUGAS (AGES): In all the four ages, the True Bani (Divine hymns) is the ambrosia.(Dhanasan M. 3, p- 665) In all the four ages, they are seiled and full of dilt, whose mouths do not utter the Name of the Lord.(Sri Raga M. l, p. 57) The Yugas (ages) are four, viz., the Kritya Yuga (or Sat Yuga), the Treta, the Dwapara and the Kali Yuga. In order to estimate the period of each Yuga, the two Ayanas compose one year. One Ayana of six months is the period of sun\’s progress north or south of the ecliptic.
The southem Ayana is a night of the gods and the northern Ayana is the day. The period of four Yugas is twelve thousand divine years; and each divine year is composed of three hundred and sixty days. The distribution of the divine years of each Yuga is as follows : Kritya Yuga : four thousand; Treta Yuga :three thousand, Dwapara Yuga : two thousand and Kali Yuga : One thousand. The period that precedes a Yuga is called Sandhya and the period which follows a Yuga is known as Sandhyasana and each one of these periods is equal to ten percent of the divine years.
Thus: Kritya Yuga lasts for 4,800 years, Treta Yuga lasts for 3,600 years, Dwapara Yuga lasts for 2,400 years, and Kali Yuga lasts for 1,200 years. In order to count the human years of each Yuga, we have to multiply the divine years by 360. In this way, Kritya Yuga lasts for 1728,800 human years, Treta Yuga lasts for 1296,000 human years, Dwapara Yuga lasts for 864,000 human years, and Kali Yuga lasts for 432,000 human years. A day of Brahma is called a Kalpa. A day of gods is equal to one human year.
The 12,000 divine years are to be multiplied by 360, in order to get the number of human years in the four Yugas. One Kalpa consists of 1,000 divine yearsi Thus a day or night of Brahma is equal to 12,000x360x1,000 human years i.e. 4,320,000,000 human years
1. Kohli,Surindar Singh ed,Dictionary of Mythological References in Guru Granth Sahib 1993