The Hindu Calendar Explained: Why Dates Change

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Every year, a few weeks before a major festival, the same conversation happens on a few million family WhatsApp groups. An aunt in Chennai says Janmashtami is on the Tuesday. A cousin in Delhi says it is the Wednesday, and adds that it is in a different month altogether. The temple in New Jersey has put up a banner for the Wednesday. Somebody’s mother-in-law, who is usually right about these things, is keeping her fast on the Tuesday. Everyone begins to suspect that somebody has made a mistake.

Nobody has. All four can be correct at once, and understanding why is one of the more satisfying things you can learn about Indian tradition — partly because the answer is genuinely elegant, and partly because it ends a decade of mild annual anxiety in about twenty minutes of reading.

The short version: there is no single Hindu calendar. There is a shared set of astronomical building blocks, several different reckonings built on top of them, two incompatible ways of deciding where a month begins, a dozen regional calendars, at least four live eras, and the inescapable fact that the moon does not care about midnight. What follows is each of those, in order.

The Panchanga: Five Limbs of a Day

The word panchanga — the almanac most Indian households still consult — literally means five limbs, and the five are the five things it tells you about any given day:

Tithi — the lunar day, the most important of the five and the one that fixes almost every festival.
Vara — the weekday, the only one of the five a Western calendar also has.
Nakshatra — which of the twenty-seven lunar mansions the moon occupies. The ecliptic is cut into arcs of 13°20′.
Yoga — the combined longitude of sun and moon, divided into twenty-seven parts of 800′ each.
Karana — half a tithi, running until the sun–moon angular distance increases by 6°. There are eleven of them: four fixed, seven repeating, filling sixty slots across a lunar month.

You do not need the last three to know when Diwali is. But their presence explains the form of the thing. A panchanga is not a grid of boxes you tick off. It is a daily report on the relative positions of the sun and moon, from which the quality of the day is read. The calendar is descriptive before it is administrative.

Why a Tithi Is Not a Day

Here is the single idea that dissolves most of the confusion.

A tithi is the time the sun and moon take to change their relative angular distance by 12 degrees. Thirty tithis make a lunar month. Because the orbits are elliptical and the moon’s speed varies, a tithi is not a fixed length: it runs anywhere from about 21.5 to 26 hours.

So a tithi almost never lines up with a civil day. It begins at some moment — 11:42 pm, say — and ends at some other moment the following evening. For practical purposes, the rule across most of India is that a day takes the tithi that is running at sunrise. And that is where everything downstream comes from.

Two consequences follow immediately. If a tithi is short and manages to begin and end entirely between two sunrises, it never holds a sunrise, and it is skipped from the calendar — a kshaya tithi. If a tithi is long and holds two sunrises, it is repeated, appearing on two consecutive days. This is why your panchanga occasionally shows no Chaturthi at all that fortnight, or two Ekadashis in a row, and why fasting rules get complicated in exactly those weeks.

It is also why the same festival can be on different civil days in different cities. Sunrise in Chennai is not sunrise in Chicago. If a tithi begins at 5:50 am local time, one city has already seen the sun and takes the previous tithi; another has not and takes the new one. Nobody is wrong. The rule was applied correctly in both places to a sky that looked different from each.

Paksha and the Twelve Months

Each lunar month has two fortnights, called paksha. The waxing half, ending at the full moon, is Shukla paksha — bright. The waning half, ending at the new moon, is Krishna paksha — dark. A festival is normally named by month, paksha and tithi: Chaitra Shukla Navami, Bhadrapada Krishna Ashtami.

The twelve lunar months, in order, are Chaitra, Vaishakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashvin, Kartika, Margashirsha, Pausha, Magha and Phalguna. Most are named for the nakshatra near which the full moon falls in them — Kartika for Krittika, Chaitra for Chitra, and so on.

Amanta and Purnimanta: The Great Divide

Now the part that causes the family argument.

Everyone agrees there are two fortnights and twelve months. Nobody agrees where a month starts.

Amanta reckoning ends the month at the new moon. The month begins the day after amavasya, so the bright fortnight comes first. This is used across the peninsula: Gujarat, Maharashtra, Goa, Karnataka, Andhra Pradesh and Telangana. The Indian national calendar follows it too.
Purnimanta reckoning ends the month at the full moon. The month begins the day after purnima, so the dark fortnight comes first. This is used in Nepal and across most states north of the Vindhyas.

The consequence is precise and worth stating carefully, because it is usually got wrong. The bright fortnight falls in the same named month under both systems. Only the dark fortnight is displaced — and in amanta reckoning it carries the month name one step earlier than it does in purnimanta.

Three worked examples make it concrete:

Maha Shivaratri is a dark-fortnight festival. Amanta: Magha Krishna. Purnimanta: Phalguna Krishna. Same night, two month names.
Diwali’s main night is the new moon. Amanta: Ashvin amavasya — which means the first four days of Diwali fall in Ashvin and only the fifth, Govardhan Puja and Bhai Dooj, falls in Kartika. Purnimanta: the whole festival sits inside Kartika from Govatsa Dwadashi onward.
Krishna Janmashtami is Krishna Ashtami. Amanta: Bhadrapada. Purnimanta: Shravana. This is the one that most often starts the WhatsApp argument.

So when your Delhi cousin says Shravana and your Hyderabad aunt says Bhadrapada, they are describing the same night. Neither has mislearned anything.

The historical account, as traditionally given, is that purnimanta reckoning was used in the Vedic period, was displaced by amanta sometime before the first century BCE, and was restored in the north by Vikramaditya in 57 BCE. Like most clean stories about calendar history, it should be held a little loosely; the textual evidence for the sequence is thinner than the confidence with which it is usually repeated.

The Extra Month: Adhika Masa

Twelve lunar months come to roughly 354 days. A solar year is about 365. Left alone, the lunar calendar would drift through the seasons the way the Islamic calendar does, and Holi would eventually arrive in autumn. Hindu reckoning refuses that, because the festivals are agricultural and seasonal before they are anything else. So the calendar is lunisolar: it counts by the moon and corrects to the sun.

The correction is an extra month. Roughly every 32.5 months — about every 2.71 years — a lunar month is doubled. The intercalation follows a nineteen-year cycle very like the Greek Metonic cycle, with the extra month falling in years 3, 5, 8, 11, 14, 16 and 19.

The doubled month is called Adhika masa (also Mala, and sometimes Asura masa), and its twin is the Shuddha or pure month (also Deva masa). The adhika month is conventionally treated as inauspicious: weddings, housewarmings and new ventures are pushed out of it. Vaishnava tradition reframes the same month warmly as Purushottama masa, a month belonging to God precisely because no worldly business claims it, and fills it with reading and fasting instead. Both attitudes are alive, and a family may well hold the first about weddings and the second about practice.

Margashirsha, Pausha and Magha are excluded from repetition. The classical texts are not entirely consistent about these rules, and it is worth knowing that — anyone who tells you the system is perfectly uniform is tidying.

The mirror of the extra month is the dropped one. A kshaya masa occurs when a new moon falls immediately before a sankranti while the sun is at perigee and the lunar month is at its longest. It is genuinely rare: such years are typically at least 141 years apart. The next one scheduled is the loss of Pausha in January 2124.

The Other Calendar Running Underneath

Alongside all of this, a solar calendar runs in parallel, and in several parts of India it is the primary one.

The zodiac is divided into twelve rashis of 30° each. The sun’s passage through one rashi is a solar month; its entry into a sign is a sankranti. This is why some festivals behave completely differently from the lunar ones: Makar Sankranti, Pongal and Vaisakhi land on nearly the same Gregorian date every year, because they are solar events, not lunar ones. If a festival moves around by two or three weeks annually, it is lunar. If it sits on 14 or 15 January every year, it is solar.

One subtlety is worth knowing, because it explains an apparent oddity. Hindu astronomical reckoning uses the sidereal zodiac, fixed to the stars, rather than the tropical zodiac fixed to the equinoxes. The two drift apart over centuries. Makar Sankranti is described in the tradition as the sun’s turn northward — the winter solstice — but now falls in mid-January, more than three weeks after the actual solstice, precisely because of that drift. Astronomers point this out regularly; traditional almanac-makers largely retain the sidereal reckoning. It is a real and unresolved disagreement, not a mistake anyone has failed to notice.

The Regional Calendars

Several parts of India keep solar calendars with their own month names, their own new year and their own era.

Tamil Nadu. A solar calendar whose months replace Mesha and Vrishabha with Chithirai, Vaigasi and the rest. Chithirai begins in April–May, and Puthandu, the Tamil new year, falls at its start. The Tamil reckoning is also why Karthikai Deepam lands on a different day from the northern Kartik Purnima most years.

Bengal. A solar calendar used in West Bengal, Bangladesh, and parts of Assam and Tripura, with the year beginning in Boishakh. Its origins are genuinely disputed. Some historians credit King Shashanka, whose reign spans 594 CE, with an era beginning around 590–600 CE. Others trace it to Akbar’s Tarikh-e-Elahi, a lunar–solar hybrid devised by Fathullah Shirazi for revenue collection; Amartya Sen holds that the Tarikh-e-Elahi was largely abandoned after Akbar but left traces in the Bengali calendar, and Shamsuzzaman Khan suggests Murshid Quli Khan may have put it to fiscal use. A third account credits Alauddin Husain Shah. The epoch is 593–594 CE either way. The practical oddity most families notice is that Pohela Boishakh falls on 15 April in India and 14 April in Bangladesh, because the two countries now keep structurally different calendars: India’s follows the Surya Siddhanta, while Bangladesh adopted a reformed fixed-length version — drafted by a committee under Muhammad Shahidullah in 1966, made official in 1987, and revised again from 2019.

Kerala. The Kollam era, or Kollavarsham, is a sidereal solar calendar conventionally dated from 825 CE. Why is an open question with at least five answers in circulation: the founding of Kollam under Maruwan Sapir Iso after Venad was freed from Chola rule; a Parashurama cycle counted in millennia from 1176 BCE; a commemoration of Adi Shankaracharya’s death around 820 CE, news of which reached Kerala some years later; the building of a Shiva temple at Kollam, per Hermann Gundert; or the founding of the Thirupalkadal temple. The months are Chingam, Kanni, Thulam, Vrischikam, Dhanu, Makaram, Kumbham, Meenam, Medam, Edavam, Mithunam, Karkidakam. Chingam 1 is the Kerala new year under this era; Vishu, the first day of Medam, was the new year before 825 CE and is still kept as the traditional Malayali one.

Odisha. A solar calendar structurally like the Bengali and Assamese ones, with Baishakha, Jyeshtha and Ashadha among its months — which is why Odia observances often sit a little differently from both the northern lunar and the southern amanta patterns.

Which Year Is It?

There is no single answer, and the honest reply to the question is in which era?

Vikram Samvat, from 57 BCE, used across northern, western, central and eastern India and in Nepal. Add 57 to the Gregorian year, broadly — 2026 CE falls in Vikram Samvat 2083.
Shaka Samvat, from 78 CE, long used in the south. Subtract 78 — 2026 CE falls in Shaka 1948. There is also an Old Shaka era whose epoch is genuinely uncertain and probably lies somewhere in the first millennium BCE.
Kollam era, from 825 CE, in Kerala.
Bengali San, from 593–594 CE.

And then there is the official one. In 1952 the Government of India set up a Calendar Reform Committee under the physicist Meghnad Saha, within the Council of Scientific and Industrial Research. It reported in 1954, recommending a single national civil calendar, and the Indian national calendar came into force on 1 Chaitra 1879 Saka — 22 March 1957. It is a fixed solar calendar: Chaitra has 30 days and begins on 22 March, or 31 days beginning 21 March in a leap year; Vaishakha through Bhadra have 31 days; the rest have 30. The leap rule is simply to add 78 to the Shaka year and check whether the result is a Gregorian leap year. You will find it in The Gazette of India, in All India Radio bulletins and on government calendars. You will not find it in many homes. Panchanga-makers and the public never took it up, which tells you something about what a calendar actually is: not a convention a committee can issue, but a habit a culture keeps.

Why Your Date Differs From Your Parents’

This is the practical section, and it matters most for readers outside India.

There are four separate reasons a festival date can differ between you and your family, and it helps to know which one you are dealing with.

The sunrise rule applied at your longitude. A tithi starts at a moment, and your day takes whichever tithi is running at your sunrise. A tithi beginning at 7:10 am IST has already begun, hours earlier in local terms, by the time the sun rises in London. The date can legitimately shift by a day.
Amanta versus purnimanta. If the disagreement is about the month name rather than the day, this is almost always it, and both parties are right.
Different panchanga computations. Almanacs differ on whether they use modern observational positions (drik) or the older Surya Siddhanta values, and on which rules they apply when a tithi is skipped or doubled. Two careful almanacs can disagree by a day.
A community decision. Diaspora temples sometimes keep a festival on the IST date, or move it to the nearest weekend so that people with jobs and school runs can actually come. This is a pastoral choice, not an astronomical one, and it is usually the right one.

The practical advice is unglamorous and works: pick one authority — a panchanga computed for your city, or the temple you actually attend — and stay with it for the year. Do not cross-check three sources the night before a fast; you will find a discrepancy, because there is one. The vrata rests on your intention and your observance, not on having adjudicated a calendrical dispute correctly. And if you are the person in the family who now understands all this, resist the urge to be right about it in the group chat on the morning of the festival.

Why This Matters Today

It would be easy to treat all of the above as charming complexity — a traditional system that has not got round to standardising. That reading misses what the calendar is actually for.

A Gregorian date tells you where you are in an accounting period. A panchanga line tells you what kind of day it is: which tithi, which nakshatra, which combination. It assumes that time has texture — that a day has a quality, not merely a number — and it ties the festivals to things you can look up and see. Krishna Ashtami is not the eighth of anything administrative; it is the eighth day of a waning moon, and you can walk outside and confirm it. Kartik Purnima is a full moon, visibly. Holi is a full moon. Diwali is the darkest night of its month, and that is the entire reason for the lamps. The system keeps the festival attached to the sky it came from.

That is also why the intercalary month exists instead of a simple drift. The tradition insisted on both things at once: a calendar read off the moon, and festivals that stay in their season. Keeping both is harder than keeping either, and the extra month is the price. There is a decent lesson in that, in a culture that generally resolves such tensions by abandoning one side of them.

And the regional multiplicity is not disorder. It is the record of a civilisation that let Kerala keep its era and Tamil Nadu its months and Bengal its new year, and never required one of them to be the real one. A committee of excellent physicists produced a single rational national calendar in 1957 and nobody much used it. The old ones are still running, all of them, every year.

Hindu Calendar FAQ

Why do Hindu festival dates change every year?

Because most of them are fixed by the lunar calendar, which is about eleven days shorter than the solar year. A festival named by month, fortnight and tithi therefore moves against the Gregorian calendar — but only within a window, because an extra lunar month is inserted roughly every 32.5 months to pull the whole system back into its season. Solar festivals such as Makar Sankranti and Pongal do not move, which is why they sit on almost the same date every year.

What is a tithi?

A tithi is a lunar day: the time the sun and moon take to change their relative angular distance by 12 degrees. There are thirty in a lunar month. Because it is defined by angle rather than by rotation, a tithi runs between roughly 21.5 and 26 hours, which is why it does not line up with a civil day and why a tithi is occasionally skipped or repeated in the almanac.

What is the difference between amanta and purnimanta?

They are two ways of deciding where a lunar month ends. Amanta months end at the new moon and are used across Gujarat, Maharashtra, Goa, Karnataka, Andhra Pradesh and Telangana. Purnimanta months end at the full moon and are used in Nepal and most of north India. The bright fortnight carries the same month name in both; only the dark fortnight is displaced, with the amanta name one month earlier. Maha Shivaratri is Magha Krishna in amanta and Phalguna Krishna in purnimanta — the same night.

What is adhika masa and is it inauspicious?

Adhika masa is the extra lunar month inserted roughly every 32.5 months to keep the lunar calendar in step with the solar year. Convention treats it as unsuitable for weddings, housewarmings and new ventures. Vaishnava tradition calls the same month Purushottama masa and treats it as especially good for fasting, reading and practice, on the grounds that a month with no worldly business in it belongs to God. Both views are widely held.

Why is my festival date different from the one in India?

Chiefly because a tithi begins and ends at a moment in time, and the rule that a day takes the tithi running at sunrise is applied at your own location. Sunrise where you live is not sunrise in Ujjain. Almanacs also differ in their computations, and diaspora temples sometimes keep the Indian date or move an observance to a weekend. Use a panchanga calculated for your city, or follow your temple, and expect occasional divergence.

Which Hindu calendar is the correct one?

There is no single correct one, and the tradition has never required one. Lunar and solar reckonings run in parallel; amanta and purnimanta are both standard; Tamil Nadu, Bengal, Kerala and Odisha keep their own solar calendars and eras; and several eras — Vikram Samvat, Shaka Samvat, the Kollam era, the Bengali San — are all current. The Government of India adopted a unified national calendar in 1957, and it is used in official gazettes while the regional calendars continue in everyday life.

A Closing Thought

There is a moment, once you have followed all of this through, when the family argument stops being annoying and becomes rather moving. Your aunt in Chennai and your cousin in Delhi are not disagreeing. They are each reporting, accurately, from inside a reckoning their part of the country has used for a very long time, about a moon that both of them can see.

The Hindu calendar never resolved its own plurality, and the striking thing is how little trouble this has caused in practice. People keep the festival on the day their tradition gives them, and on that day the moon is doing exactly what the almanac says it is doing. That is a strange and durable kind of accuracy: not everyone agreeing on the number, but everyone looking at the same sky.

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