Next birthday calculator
A leap-day birthday has no date in three years out of four. The convention used here — and by most legal systems and almost all software — is that the anniversary falls on 1 March in a common year. Some countries and some people prefer 28 February; nothing in the calendar decides it, so the tool says which one it picked.
Counts the days from today to your next birthday, names the weekday it falls on, and says which age you will turn. Leap-day birthdays are handled explicitly rather than silently.
How to use the next birthday calculator
The interesting question a countdown answers is which day of the week it lands on, because that decides whether it is worth taking the day off. The weekday shifts by one each year and by two across a leap year, which is why a birthday that fell on a Saturday will not do so again for six or eleven years.
The genuinely awkward case is 29 February. Three years in four it does not exist, and no calendar rule decides what to do — it is a legal and social convention, and different jurisdictions chose differently. In England and Wales a person born on 29 February legally turns 18 on 1 March; in New Zealand it is 28 February. This uses 1 March, which is the majority convention and what almost all software does, and it says so on the page rather than quietly picking.
One thing worth knowing about counting: the answer is in whole calendar days in your own time zone, not in 24-hour periods. Those differ by an hour twice a year where clocks change, which is exactly why the calculation is done on dates rather than on timestamps.
Questions
In a common year the anniversary is taken as 1 March, which is the usual legal convention and what most software does. Some countries use 28 February.