How anniversary dates are calculated
The calculator takes the original event date and adds whole years to find past and future anniversary dates. The next anniversary is the smallest future date that falls on the same month and day as the original event, in any upcoming year. The countdown is the number of calendar days from today to that next date.
If the original event fell on 29 February (a leap day), the calculator applies a configurable rule: either observe the anniversary on 28 February in non-leap years, or on 1 March. The default is 28 February, which is the convention used in most legal and civil contexts. The years-elapsed count is simply the current calendar year minus the event year, adjusted if today's date is before the anniversary date in the current year.
Years elapsed = Current year − Event year (minus 1 if today is before this year's anniversary)
The number of completed anniversary cycles.
Countdown (days) = Next anniversary date − Today's date
The number of days until the next occurrence.