Why panchang sites show different festival dates

The moment of the day that decides

A tithi can begin at any hour, so most civil days hold two of them. A festival is therefore tied to a moment of the day at which its tithi has to be in force. In our calendar that is sunrise for most fasts, the afternoon (Aparahna) for Bhai Dooj, the evening after sunset (Pradosh) for Diwali's Lakshmi Puja, the middle of the night (Nishita) for Kali Puja, and moonrise for Karva Chauth. Two calendars that check different moments for the same festival will often name different days. Every festival page on this site states the moment it uses.

Source for the rule: Dharma Sindhu (Kashinatha Upadhyaya), pariccheda 2, Kartika, Yama Dwitiya (Chaukhamba Sanskrit Pratishthan edition, Delhi 1994, p. 161); the Aparahna as the fourth fifth of the day: Nirnaya Sindhu (Kamalakara Bhatta), pariccheda 2, Kartika (Nirnaya Sagar Press edition, Bombay 1940, p. 150).

Sunrise is local, so the city can change the day

A tithi begins and ends at the same instant everywhere, but the sun rises earlier in the east of India than in the west. A tithi that begins shortly before dawn can be in force at sunrise in one city and miss sunrise in another, so the same rule gives different days. A calendar worked out for New Delhi and read in Kolkata or Chennai can be a day out. Our dates are worked out for each city's own sunrise, sunset and moonrise.

Lost and repeated tithis (kshaya and vriddhi)

A tithi is not a day long. Now and then one begins after a sunrise and ends before the next, so it is in force at no sunrise at all: a kshaya, or lost, tithi. Another lasts across two sunrises: a vriddhi, or repeated, tithi. A rule that looks for the tithi at sunrise then finds no day, or two, and each calendar has to say what it does. Since sunrise is local, the same tithi can be lost in one city and not in another.

Smarta and Vaishnava Ekadashi

In some months a calendar prints two Ekadashi dates, one for Smarta households and one for Vaishnavas. The Vaishnava date in our calendar adds a test at Arunodaya, the last part of the night before sunrise: if Dashami is still running then, or if Ekadashi and Dwadashi both run past the next sunrise, the Vaishnava fast moves to the following day. So the two fasts share a day in most fortnights and stand a day apart in a few. A site that prints one date without saying which it is will disagree with some of its readers.

These tests describe what our calculation does. The record of the texts behind the Smarta and Vaishnava tests is not yet complete; this page will cite it when it is.

Purnimanta and amanta month names

Some calendars end the lunar month at the full moon (purnimanta), as is usual in North India; others end it at the new moon (amanta), as in much of western and southern India. The two agree on the month of the bright fortnight and differ by one month in the dark fortnight. The date of a festival does not change, only the name of its month. If one source says Kartika and another says Ashwin for the same day, check the reckoning before assuming they disagree.

The extra month (adhik maas)

Twelve lunar months fall short of a solar year, so every two or three years a lunar month passes in which the Sun enters no new sign. That month is counted twice: first as the adhik (extra) month, then as the regular one. A calendar has to say which of the two runs of tithis each observance of that month belongs to, and one that does not insert the extra month will name the months that follow wrongly.

Bhadra and Holika Dahan

The Holika bonfire is lit in the evening of Phalguna Purnima, and not in Bhadra (Vishti karana). When Bhadra runs late into the night, the texts give more than one answer for which evening to use, and calendars choose between them. Two sites can then be a day apart and both be following a written rule.

Sources: Dharma Sindhu, pariccheda 2, Phalguna (Chaukhamba edition, pp. 195-197), "सा प्रदोषव्यापिनी भद्रारहिता ग्राह्या"; Nirnaya Sindhu, pariccheda 2, Phalguna, Holika-nirnaya (Khemraj Shrikrishnadas edition, pp. 341-344), which records both the later-evening view and the earlier-evening view that only Bhadra's mouth is avoided.

One clock, many skies

India keeps one clock, IST, across a wide country, so a moment fixed by the sky arrives at different clock times in different cities on the same date. A puja time copied from another city's calendar does not apply in yours. Abroad, the date itself is reckoned in local time, so a festival can fall on the day before India's.

How sunrise and moonrise are measured

Calendars also differ in what they call sunrise and moonrise. Our sunrise is the moment the Sun's upper edge appears, with standard refraction. Our moonrise is the moment the centre of the Moon's disc reaches the horizon, with no refraction; calendars that print the moment the Moon's upper edge first appears show a time about 4 to 5 minutes earlier for Indian cities, and more at the latitude of London or Toronto. Differences in a city's stored coordinates and in the refraction value move sunrise by tens of seconds. These rarely change a date, but they decide the close cases: a tithi that ends within a few minutes of sunrise can fairly be given to either day.

From our methodology and the moonrise note on Karva Chauth moonrise by city.

The ayanamsa

The ayanamsa is the correction from the tropical to the sidereal zodiac, and calendars use different values. Ours is Lahiri (Chitrapaksha), the Rashtriya Panchang standard. A tithi is the angle between the Moon and the Sun, so the ayanamsa cancels out of it: every ayanamsa gives the same tithi at the same moment. It does move whatever is measured against the stars, such as the nakshatra and the Sun's entry into a sign. The Lahiri and Raman values differ by about 1°27′, which the Sun takes more than a day to cover.

How to check a date yourself

1. Make sure both calendars are set to the same city. A date or time for New Delhi is not a date or time for Patna, Pune or Toronto.
2. Find the moment the festival is decided at: sunrise, afternoon, evening, midnight or moonrise. Each of our festival pages says which.
3. Look up the tithi at that moment on both days, not just the tithi printed against each day, which is usually the tithi at sunrise.
4. For an Ekadashi, check whether the date given is the Smarta or the Vaishnava one.
5. If the month names differ, check whether one calendar counts months purnimanta and the other amanta.
6. Outside India, use the local date and time, not IST.
7. If the dates still differ, write to us at [email protected] with the city, the festival and the source you compared. Every report is checked against the calculation, and the reason for each known difference is listed in the accuracy record.