Numerous calendars are used in India for different purposes. The Gregorian calendar is used by the government for civil matters; the Islamic calendar is used by Muslims; the Hindus employ both solar and lunisolar calendars. Indeed, there are over 30 variations of the Hindu calendar in active use. In March 1957, an attempt was made to revise the traditional calendar to follow the pattern of the Gregorian leap-year structure [1]. The proposed reform has not, however, been widely accepted, though the new National Calendar dates appear in published calendars. An excellent description of many Indian calendars, together with extensive tables, is given in [6].

The best known of several related systems used on the Indian subcontinent is the classical Hindu calendar of the (present) Sūrya-Siddhānta (circa 1000), said to have been revealed to Asura Maya the Assyrian at the end of the last “Golden Age,” in the year 2163154 b.c.e. al-Bīrūnī attributes the book to Lata. This work introduced a calendar based on approximations to the true times of astronomical events rather than the mean values used in the earlier, simpler, calendar described in Chapter 10. This calendar is somewhat similar to the Chinese, beginning its months according to the actual time of new moon; however, the Chinese calendar today uses modern astronomical methods to determine these times whereas the Hindu calendar applies fixed, ancient methods to approximate the true positions of the sun and moon.