Nepali Calendar uses lunar months and solar sidereal year. The New Year is on 1st Baishakh (mid of April). It is approximately 56 years and 8½ months ahead of the Gregorian calendar (A.D).
Eastern Hindu lunar calendar is 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. Hence an extra month, called the Purushottam month (Adhik Mas), is inserted every 3 years.