What is a Hindu birthday, or janma tithi?
The janma tithi (जन्म तिथि) is the lunar day on which a person was born: one of the thirty steps of the Moon between one new moon and the next, in a named lunar month and in its bright or dark half. The Hindu birthday is the day that same tithi comes round again in the same month, and the old books call its observance vardhapana or varsha-vriddhi, the growing of the years. Rama's birthday is kept this way on Chaitra Shukla Navami and Krishna's on the dark Ashtami of Shravana (Bhadrapada by the northern count), which is why Ram Navami and Janmashtami fall on a different calendar date each year. A family that keeps tithi birthdays does for its own members what the calendar does for them.
How is the janma tithi birthday date decided?
First the month: the lunar month of the birth, counted from new moon to new moon, with the same bright or dark half. Then the tithi inside it. A tithi rarely fits a civil day; it usually begins on one day and ends on the next, so a rule has to choose. The Dharmasindhu, the manual most North and West Indian pandits work from, sets the order of the tests in its section on vardhapana, and the Nirnayasindhu agrees that the tithi at sunrise is the one that counts for a birthday. This calculator applies those tests in that order and prints, under each date, which of them settled it.
The birth nakshatra is asked first
When the tithi lies across two days, the Dharmasindhu takes the day on which the birth nakshatra is also present with it. The rite began as a nakshatra observance in the older grihya texts, and this clause keeps that link. If the star joins the tithi on both days, or on neither, the question passes to the sunrise test below. In practice the star settles the matter in a minority of years, and the result line says so when it does.
Two muhurtas past sunrise is the sunrise test
Otherwise the day is the one whose sunrise falls inside the tithi, provided the tithi runs on for more than two muhurtas, 96 minutes, after that sunrise. If it ends sooner, the day before is taken, since the tithi filled most of that day. Sunrise differs from city to city, so two places far apart can keep the same person's birthday a day apart in the same year, and both are right for where they stand.
A kshaya tithi holds no sunrise at all
Now and then a tithi begins after one sunrise and ends before the next, so no sunrise falls inside it. Almanacs call this a kshaya tithi. The birthday is then kept on the day the tithi begins, which is the only day it touches. The opposite case, a long tithi that holds two sunrises, goes through the same two tests as any other, with the second of the two days as the sunrise day.
What if you were born in an adhika masa?
About once in thirty-two months the lunar calendar adds a month, the adhika masa, to stay in step with the Sun. It carries the name of the month that follows it, so a year can hold an extra Shravana before the regular one. The Dharmasindhu is direct about birthdays: a person born in an adhika month keeps the yearly observance in the regular month of that name, not in the added one. This calculator does the same, and tells you when your birth fell in an added month. In a year when your month comes twice, the date shown is in the regular month.
What is a star birthday, or janma nakshatra birthday?
In Kerala and Tamil Nadu the birthday is kept by the birth star, not the tithi. The janma nakshatra is the one of the twenty-seven star divisions the Moon stood in at birth, and the star birthday is the day the Moon returns to it during the solar month of the birth: the Malayalam or Tamil month, which runs from one sankranti to the next. This is regional custom carried by the almanacs, not a rule from the nibandhas, and the page marks it so. The Moon passes every star about once in 27 days, so the star always comes at least once in the month.
The second occurrence when the star comes twice
A solar month is two to four days longer than the Moon's round of the stars, so the birth star sometimes falls at both ends of the month. Kerala and Tamil practice then takes the second occurrence. The calculator follows that, and shows the star's start and end times so that a family with its own custom can see the other day as well.
Six nazhika of the star after sunrise
Kerala almanacs ask that the star last at least six nazhika, about two hours and twenty-four minutes, past sunrise on the day; when less than that remains, the previous day is taken. A nazhika is a sixtieth of a day, and the measure is scaled to a tenth of the star's own length. Tamil sources give the same month-and-star rule without stating a minimum, so the Kerala measure is used for both and named as such.
What is done on a tithi birthday, and what is avoided?
The Dharmasindhu's order of the day is short. A bath with sesame, after rubbing the body with sesame paste; new clothes; worship of the family deity, the birth star, one's parents, and the long-lived ones, with the sage Markandeya first among them; then a sip of milk mixed with sesame and jaggery, taken with a verse that asks for years as long as his. Food is given to others before the day ends. The same texts list what to leave alone on the day: cutting hair or nails, travel, meat, quarrels and harm to any creature, and they advise against a hot-water bath. Lamps, cake and the homa many families add are later custom, welcome but not from this section.
How far does the Hindu birthday move from the calendar date?
Twelve lunar months come to about 354 days, eleven short of the solar year, so the tithi birthday falls about eleven days earlier each year until an added month pushes it back by roughly nineteen. Over a life it stays within about a month on either side of the calendar birthday. The two meet on the same day only a few times in a lifetime, most often near the ages of 19, 38, 57 and 76, because 235 lunar months equal nineteen solar years to within about two hours. The result lists the years in which your two birthdays coincide, worked out for your own birth.
What are Shashtiabdapurti and Sahasra Chandra Darshan?
They are the milestone birthdays of the Hindu reckoning. Shashtiabdapurti marks the completion of sixty years, the full round of the sixty named years, and is kept in the birth month on the birth star. Sahasra Chandra Darshan, the sight of a thousand moons, honours a person who has lived through a thousand full moons. A thousand lunations take a little under eighty-one years; one old grihya tradition states it as eighty years and eight months. This page counts the full moons since your birth and gives the month in which the thousandth falls, so a family can plan for a parent or grandparent.
How this is calculated
Nirayana rashis with the Lahiri (Chitrapaksha) ayanamsha and whole-sign bhavas, computed to arc-second accuracy. Sunrise and sunset are the ones almanacs show, the Sun's upper edge with refraction, so the daily windows agree with the calendars you compare them with. More on our method.