Three ways to keep time
Almost every calendar in the world is built from three natural cycles: the day, the month of the Moon’s phases (about 29.5 days), and the year of the seasons (about 365.24 days). The trouble is that none of them divides evenly into another. Every calendar is a different answer to the same puzzle:
- Solar calendars follow the year and let the months drift from the Moon. The Gregorian calendar, the Indian national (Shaka) calendar, the original Nanakshahi calendar and the Fasli Parsi calendar work this way.
- Lunar calendars follow the Moon and let the year drift through the seasons. The Hijri (Islamic) calendar is purely lunar, so its dates move about 11 days earlier each Gregorian year.
- Lunisolar calendars follow the Moon but add an extra month every few years to stay with the seasons. The Hindu panchang and the Jain calendar work this way, with Adhik maas as the extra month.
The Shahenshahi and Kadmi Parsi calendars are a fourth kind: a fixed 365-day year with no leap day, which slowly moves through the seasons.
Why the same day has different dates
A single day in India can be, at the same time, a date in the Gregorian calendar, a tithi in the panchang, a day in the Hijri month, a Roj and Mah, and a Nanakshahi date. Festivals fixed in one calendar move in the others, which is why Diwali, Eid, Navroz and many gurpurabs fall on different Gregorian dates each year.
Within one community, dates can also differ, for good reasons:
- Location. Sunrise, sunset and moonrise depend on where you are, and many rules are tied to them.
- Method. Amanta or Purnimanta months; a particular prayer-time convention; Shahenshahi, Kadmi or Fasli; the original or SGPC Nanakshahi.
- Observation. Many Islamic months begin with the sighting of the crescent, announced locally.
- Custom. Communities and families may follow their own rule for which day to observe.
OIOA calculates the astronomy for your location and the method you choose. For the day your family observes, please follow your community’s authority.
The calendars
- Hindu panchangThe Hindu panchang explained: a lunisolar calendar of tithis, nakshatras and lunar months, kept in step with the seasons by Adhik maas. What OIOA computes.
- Vikram SamvatWhat Vikram Samvat is, how to convert its years to the Common Era, why its new year falls in Chaitra in some regions and after Diwali in Gujarat.
- Shaka SamvatShaka Samvat explained: the era used across India and the basis of the national calendar, with Chaitra 1 on 22 March, and how to convert years.
- Jain calendarHow the Jain calendar works: the Vira Nirvana Samvat era, a lunisolar year of tithis, 28 nakshatras, and why sects may observe days differently.
- Hijri (Islamic) calendarThe Hijri calendar explained: 12 lunar months, moon sighting, Umm al-Qura, and the Shia and Bohra (Dawoodi, Misri) options OIOA supports.
- Parsi calendarsThe Zoroastrian calendar explained: 12 months of 30 days and 5 Gatha days, named Roj and Mah, in the Shahenshahi, Kadmi and Fasli versions.
- Nanakshahi calendarHow the Nanakshahi calendar works: its era, months from Chet to Phagun, Sangrand, and the original 2003 version versus the SGPC method.