Why two calendars differ One day, two places, and the rule that moves a Tamil date by twenty-four hours: which Sun the month is reckoned from, which clock the ingress landed on, and which side of sunset it fell.

Choose a date to compare the two places on.

A month begins when the Sun changes sign

A Tamil month is not a length of time but a place in the sky: Chithirai is the month the Sun spends in Mesham, Vaikasi the month it spends in Rishabam, and so on round the twelve. So a month begins at a sankramana, the instant the Sun crosses from one sidereal sign into the next. That instant is the same for everybody alive. What differs is the calendar it is written into.

First rule: which Sun

Tamil Nadu has two Suns in print, and both are honestly arrived at. The thirukanitha reckoning takes the Sun as modern astronomy computes it, which is what Drik Panchang, Prokerala and the Chennai daily calendars print. The vakya reckoning takes the Sun of the classical Vakyakarana tradition, whose year length and equation of centre come down from Aryabhata and are carried in memorised Sanskrit sentences rather than computed; this is the Pambu panchangam and most of the printed wall calendars. The classical year is about four minutes too long, and sixteen centuries of that has put the vakya Sun some hours behind the sky. Whenever that lag carries an ingress across sunset, the vakya month starts a day later. Over 2025 to 2027 the two agree in twenty-five months of thirty-six and differ by exactly one day in the other eleven.

Second rule: which clock

The sankramana is one instant, but it is written into a different hour in every town. An ingress at half past midnight in Chennai is nine in the evening of the day before in Frankfurt, so the two towns are not even looking at the same civil date when they apply the rule. Nothing is in dispute here: the month simply turned on a different day of the week where you are standing, the way a birthday does.

Third rule: which side of sunset

The Tamil rule is that the ingress day is day one only if the Sun entered the sign before local sunset; enter it after sunset and the month waits for the following day. Two towns can therefore take the very same instant and land on opposite sides of their own sunsets. Kerala keeps the same shape of rule but draws the line earlier, at aparahna — three fifths of the daylight after sunrise, around one in the afternoon in Kochi rather than a quarter past six. An ingress in the middle of that afternoon starts the Malayalam month a day later than the Tamil one in the same street, off the same Sun.

So who is right

Both are. A calendar is a rule for naming days, not a claim about the sky, and the three rules above are each older than the argument. What a family actually has to settle is which observance to keep: the hour of a rite is local by every tradition — an oil bath before sunrise means before the sunrise outside your own window — while the day a festival falls on is usually kept with the household you are keeping it with. Where the two answers differ, this page says which rule did it, so the telephone call can be about the choice rather than about who has the wrong calendar.

Dates and instants here come from the same engines as the rest of the app, and the vakya Sun is a model of a tradition that different almanac houses round differently: ours reproduces 34 of the 36 published Tamil month starts of 2025 to 2027, with the two it misses named in the open test file. Where your own almanac differs, its reckoning is the one your family keeps.