Two birthdays
Most families in India keep two birthdays: the Gregorian date, and the janma nakshatra birthday — the day each year when the moon returns to the nakshatra it occupied at birth, within the same lunar month.
The nakshatra birthday is the one used for rituals: an Ayushya Homa for long life, an abhisheka at the family temple, or simply a special meal and the elders' blessing. Because it follows the lunar calendar, it falls on a different Gregorian date each year — typically 11 to 19 days earlier than the previous year, then jumping forward when an adhika masa intervenes.
How this app computes it
When you add a family member with their date of birth, time of birth and birth place, we compute the moon's exact position at that moment. That gives the janma nakshatra, the pada, and the janma rashi. We then create a recurring calendar event that resolves to the correct Gregorian date each year, for the next two years ahead.
The time of birth matters. Without it we assume noon, which is accurate to the correct nakshatra roughly 85% of the time — the moon moves through one nakshatra in about a day. Adding the exact time makes it precise, and is required for the pada, which determines the naming syllable.
What families book on it
Ayushya Homa is the classic observance, especially for children and for the 60th (Shashtiabdapoorthi) and 80th (Sahasrachandra Darshana) years. Many families book a simple Satyanarayana Pooja or an archana in the person's name.
Shraddha dates work the same way
A death anniversary is observed on the tithi, not the Gregorian date. Record the tithi once and the calendar resolves it each year.