Illustrative example. The dates and amounts below are provided for practice.
Being born in a leap year does not automatically require a special birthday policy. The fallback rule matters when the birth date itself is February 29.
Enter March 1, 2024 as Date of Birth and March 1, 2025 as Calculate Age As Of. The result is exactly one year and 365 elapsed days. Changing the Leap Day Rule leaves this result unchanged.
Locate the extra day
February 29, 2024 was before this birth date, so it lies outside the measured interval. A nearby comparison makes that visible. From February 28, 2024 to February 28, 2025, the elapsed total is 366 because the leap day lies inside that interval.
The two people would each have one completed calendar year on their respective birthdays while having different total days lived.
Avoid a misleading shortcut
Count from the actual birth date rather than adding an extra day simply because “2024 was a leap year.” Inclusive numbering is a separate choice. March 1, 2025 is Day 366 when March 1, 2024 is Day 1, while 365 days have elapsed.
Use the full calculator result when comparing ages so an unexplained day-count difference is not mistaken for a wrong birthday.