A countdown to a recurring yearly date, a birthday, wedding anniversary, or work anniversary, is a slightly different calculation than a countdown to a one-time event, because the target date has to be recalculated every year rather than staying fixed. The core logic is simple in the common case, but one specific edge case, a birth date of February 29, needs an explicit rule to handle correctly.
Finding the next occurrence
The calculation starts by taking the original month and day (say, June 15) and applying it to the current year. If that resulting date is still in the future relative to today, that's the target for the countdown. If it's already passed this year, the calculation rolls forward to next year instead, using the same month and day. This is exactly why a countdown to "your birthday" automatically resets to a full year away the day after your birthday passes, rather than counting down to a date that's already gone.
The February 29 problem
Someone born on February 29 has a genuine problem: that exact calendar date only exists in leap years, roughly once every four years, so a countdown calculation that naively looks for "February 29 of next year" will fail three years out of four, since that date simply doesn't exist on the calendar for those years. The near-universal practical fix is to observe the anniversary on February 28 in non-leap years (some people and some legal systems instead use March 1, treating the missing day as if it were skipped forward rather than back), and revert to the true February 29 whenever the current year actually is a leap year.
This is directly connected to the leap year rule covered in our guide on why leap years exist: the calculation needs to check, for whichever year is being targeted, whether that specific year satisfies the leap year rule (divisible by 4, with the century-and-400 exception) before it can know whether February 29 is even a valid date to count toward. The Anniversary Countdown Calculator applies this fallback automatically, so a leap-day birthday still produces a sensible yearly countdown.
Once you have the target date, the countdown is straightforward
After the correct target date for this year (or next year) is determined, the actual countdown, days, hours, minutes remaining, works exactly like a countdown to any other fixed future date, which is covered in more general terms in our guide on counting down to a future date, including why anchoring the target to a specific time zone matters if the countdown needs to be precise down to the hour rather than just the day.