Indian Calendar - An Introduction
The western calendar is based on the sun, in which a year is the time required for the earth to complete one orbit around the sun. This precisely measures 365 days 5 hours 48 minutes and 46 seconds. The Indian calender is ingeniously based on both the sun and the moon; it uses a solar year but divides it into 12 lunar months. A lunar month is precisely 29 days 12 hours 44 minutes and 3 seconds long. Twelve such months constitute a lunar year of 354 days 8 hours 48 minutes and 36 seconds. To help the lunar months coincide with the solar year, the practice of inserting an intercalary (extra) month arose. So 60 solar months = 62 lunar months. Hence an extra month, called the Adhik Mas, is inserted every 30 months i.e. every 2.5 years.