What a Hindu birthday is
Most Hindu families that keep a traditional birthday do not use the date on the Gregorian calendar. They use the tithi, the lunar day, and the lunar month in which the child was born. The birthday comes when that same tithi returns in that same month, which is why it falls on a different Gregorian date each year. A lunar year is about eleven days shorter than a solar year, so the date moves earlier and then jumps back when an extra month, the Adhika masa, is added.
This tool finds your birth tithi and month from your birth details, then finds the date on which that tithi falls in the years you ask for, at the place where you will keep it. Many families use it to plan a puja, a fast or a family gathering on the right day.
How the answer is worked out
The tool first finds the moment of birth in universal time, using the time zone in force at the birth place on that day. It reads the Sun and Moon from the Swiss Ephemeris tables on this site. The tithi is the angle between the Moon and the Sun, divided into thirty steps of 12 degrees, fifteen for the bright half and fifteen for the dark half. The lunar month comes from the new moons: the amanta month runs from one new moon to the next, and its name follows the Sun’s sign at the new moon that opens it.
For each year you ask for, it searches that lunar month for the tithi and works out the sunrise at your chosen place. By the common rule, a tithi belongs to the day on which it is present at sunrise, the udaya tithi. When it covers two sunrises, the first day is shown and the second is mentioned. When it begins and ends between two sunrises, it is at no sunrise, and the tool says so and gives the day it begins.
Which tithi counts as yours
There are two common choices, and the tool lets you pick. The first is the tithi at the moment of birth, which matches the janma tithi in a horoscope. The second is the tithi at sunrise on the birth date, which matches the way a daily panchang names the day. For most people the two agree. They differ when the tithi changed between sunrise and the hour of birth, and your family or priest may have a preference. If you do not know the time, the sunrise rule is used.
Where traditions differ
Three points vary from family to family. Some use the purnimanta month, in which the dark half is counted with the following month, and the tool shows both names. The day is the same, only the month name differs for the dark half. Families born in an Adhika month often keep the birthday in the regular month of the same name, and the tool does the same and says so. Finally, a tithi that begins late in the day can be kept on either side, and your own custom is the right one to follow. If you are unsure, ask the person who keeps the family panchang.
Why the place matters
The tithi is the same everywhere at the same instant, but sunrise and the clock are not. A tithi that is present at sunrise in Delhi can have already ended at sunrise in Sydney, so the date can be a day apart. The tool uses the place where you will keep the birthday, which can differ from the birth place. It also converts times to the local clock, daylight saving included.
For related reading see the panchang hub, the guides to amanta and purnimanta months and Adhika masa, and the janma panchang calculator for everything about the birth moment. For festival dates in any city, use the festival dates tool.
Frequently asked questions
↳ Why does my Hindu birthday fall on a different date every year?
It follows the lunar calendar, whose year is about eleven days shorter than the solar year. The extra month added every two or three years brings it back, so the date wanders within a range of about a month.
↳ Which tithi should I use if I was born close to a tithi change?
Use the one your family follows. The tool shows the exact start and end of the tithi, so you can see how close the birth was to the change and discuss it with the family priest.
↳ What if my birth was in an Adhika month?
Many families keep the birthday in the regular month of the same name. Some keep it again in the Adhika month when one comes round. The tool shows the regular month and says when an Adhika month occurs.
↳ Why do two websites give different dates?
They may use different rules for the day: the tithi at sunrise, at midday or at the hour of birth. They may also use different places. This tool states its rule, and a place and a rule can be changed above.
↳ Does the tool work for someone born abroad?
Yes. Choose the birth place, and the time zone at that place on that date is used. Then choose the city where the birthday will be kept.