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The Indian national calendar (sometimes called Saka calendar) is the official civil calendar in use in India. It is used, alongside the Gregorian calendar, by the Gazette of India, news broadcasts by All India Radio, and calendars and communications issued by the Government of India. Beside such formal purposes, the calendar is not very widely used.^{[citation needed]}

The term may also ambiguously refer to the Hindu calendar, and the Saka era is commonly used by different calendars as well.

In leap years, Chaitra has 31 days and starts on March 21 instead. The months in the first half of the year all have 31 days, to take into account the slower movement of the sun across the ecliptic at this time.

The names of the months are derived from older, Hindu lunisolar calendars, so variations in spelling exist, and there is a possible source of confusion as to what calendar a date belongs to.

Years are counted in the Saka Era, which starts its year 0 in the year 78 of the Christian Era. To determine leap years, add 78 to the Saka year - if the result is a leap year in the Gregorian calendar, then the Saka year is a leap year as well.

The calendar was introduced by the Calendar Reform Committee in 1957, as part of the Indian Ephemeris and Nautical Almanac, which also contained other astronomical data, as well as timings and formulae for preparing Hindu religious calendars, in an attempt to harmonise this practice. Despite this effort, local variations based on older sources such as the Surya Siddhanta may still exist.

Usage officially started at Chaitra 1, 1879 Saka Era, or March 22, 1957. However, government officials seem to largely ignore the New Year's Day of this calendar in favour of the religious calendar [1].

The Reform Committee also formalised a religious calendar, referred to as the Rashtriya Panchang. This, like many regional calendars, defines a lunisolar calendar based on the authoritative version of the Surya Siddhanta from the 10th century.^{[citation needed]}

The word panchang is derived from the Sanskrit panchangam (pancha, five; anga, limb), which refers to the five limbs of the calendar: the lunar day, the lunar month, the half-day, the angle of the sun and moon, and the solar day.^{[citation needed]}

In the Rashtriya Panchang, months are determined based on the sun's position against the fixed stars at sunrise, computed by antipodal observations of the full moon. This sidereal computation avoids fixed leap year rules, but the number of days in any given month can vary by one or two days. Conversion of dates to the Gregorian calendar, or computing the day of the week, requires one to consult the ephemeris. The lay person therefore relies on the panchangs or almanacs produced by authoritative astronomical schools.^{[citation needed]}

Over time, different Brahminical bodies producing the panchangs have varied in their geographical center and other aspects of the computation, resulting in a divergence of a few days in the different regional calendars. Even within the same region, there may be more than one competing authority, occasionally resulting in disagreement on festival dates by as much as a month. The Rashtriya Panchang seeks to resolve such differences.^{[citation needed]}

• Hindu calendar
• Saka era
• Bangabda, a related Indic calendar

• Mapping Time: The Calendar and its History by E.G. Richards (ISBN 0-19-282065-7), 1998, pp. 184-185.

• Calendars and their History (by L.E. Doggett)
• Indian Calendars (by Leow Choon Lian, pdf, 1.22mb)
• Positional astronomy in India
• India Image website
• Hindu Panchang