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.