The Puranas describe a number of time cycles within cycles. Discussions of these cycles can become confusing because different cycles are measured in different types of units. For example, the cycles are often described in units of deva years ( years in the higher planets ), each of which equals 360 human years.
The following description starts with the smaller cycles and works up to the larger ones. The length of each cycle is given in ordinary human (earth) years, as well other units where appropriate. Large numbers are described using the conventions of American English: thus, a million is a thousand thousand, a billion is a thousand million, a trillion is a thousand billion.
The smallest cycle is called a maha yuga. A maha yuga is 4,320,000 human years. Each maha yuga is subdivided into the following four ages, whose lengths follow a ratio of 4:3:2:1:
Satya Yuga (also called Krita Yuga)
This first age is 1,728,000 human years. Also known as the Golden Age or age of Truth. The qualities of this age are: virtue reigns supreme; human stature is 21 cubits; lifespan is a lakh of years, and death occurs only when willed.
This second age is 1,296,000 human years. Also known as the Silver Age. The qualities of this age are: the climate is three quarters virtue and one quarter sin; human stature is 14 cubits; lifespan is 10,000 years.
This third age is 864,000 human years. Also known as the Bronze Age. The qualities of this age are: the climate is one half virtue and one half sin; lifespan is 1,000 years.
The fourth and last age is 432,000 human years. Also known as the Iron Age. This is the age in which we are presently living. The qualities of this age are: the climate is one quarter virtue and three quarters sin; human stature is 3.5 cubits; lifespan is 100 or 120 years.
Brahma Days (Kalpas)
A kalpa is a single daytime period in the life of Brahma, the creator god. Two kalpas are a day and a night of Brahma.
Each kalpa is composed of 1,000 maha yugas. A kalpa is thus equal to 4.32 billion human years.
At the end of Brahma’s daytime period, the Three Worlds (Bhuloka, Bhuvarloka, Swarloka) and the seven underworlds (of the nagas) are temporarily dissolved (pralaya); that is, the same souls will be reincarnated when the next day of Brahma begins.
The Vishnu Purana states that at the end of the daytime period of Brahma, a dreadful drought occurs that lasts 100 years, and all the waters are dried up. The sun changes into seven suns, and the three worlds (Bhurloka or Earth, Bhuvarloka or the lowest heaven, and Svarloka or the next higher heaven) and the underworlds are burned bare of life. The inhabitants of Bhuvarloka and Svarkloka flee to the next higher heaven, Maharloka, to escape the heat; and then to the next higher heaven, Janaloka.
Then mighty clouds form and the three worlds are completely flooded with water. The lord Vishnu reposes on the waters in meditative rest for another whole kalpa (4.32 billion years) before renewing the creation.
The destruction that takes place at the end of a daytime of Brahma is referred to as naimittika, which is incidental or occasional. The characteristic of this destruction is that the three worlds continue to exist but are made uninhabitable. The souls of individuals also continue to exist to be reincarnated in the next daytime of Brahma.
A year of Brahma is composed of 360 day/night cycles of Brahma, or 720 kalpas, or 8.64 billion human years.
The lifespan of Brahma is 100 Brahma years, or 72,000 kalpas, or 311.04 trillion human years.
At the end of the life of Brahma, all worlds are completely dissolved (mahapralaya).
Another cycle that overlaps the others is that of manvantaras. Each kalpa is reigned over by a succession of 14 Manus, and the reign of each Manu is called a manvantara. A single manvantara is approximately 71 maha yugas.
Each Manvantara is followed by a Deluge, which destroys the existings continents and swallows up all living beings, except the few who are preserved for the repeopling of the earth.”
We are located in the fifty-first Brahma year of the life of our Brahma.
Within that Brahma year, we are in the first Brahma day, called the Varaha kalpa.
Within that Brahma day, we are in the seventh manvantara, and in the 28th maha yuga of that manvantara. This would place us at about the 454th maha yuga of the 1,000 maha yugas that comprise this day of Brahma.
Within this maha yuga, we are in Kali Yuga. The 5100th year of Kali Yuga will correspond to the year 2,000 A.D. That means that we are fairly early in Kali Yuga and this age will continue 426,000 more years.