Formula
The next birthday is the next calendar date with that month and day, on or after the date you start from:
Days = next birthday − start date
February 29 moves to the next leap year when the coming year has no February 29.
Assumptions
- The birthday repeats every year on the same month and day.
- The start date is a calendar date.
Worked example
From October 14, 2026, a March 15 birthday is 152 days away.
Input definitions
- Birth month and day
- The anniversary. The year of birth is not required.
- Count from
- The date the countdown starts. A birthday on that date is 0 days.
Limitations
This does not give the person's age. The age calculator uses a full date of birth.
Questions
What happens on February 29?
In a year without that date, the countdown uses the next February 29.
Is the birthday today?
The result is 0 days when the start date is the birthday.