Elapsed time still follows the calendar
Elapsed days do not require an invented February 29 in a common year. The civil sequence simply moves from February 28 to March 1. For example, February 29, 2024 through February 28, 2025 contains 365 elapsed date boundaries. Choosing an anniversary convention changes when a completed calendar year is displayed; it does not rewrite the underlying sequence or the separately calculated total days.
February 28 is one convention
With the February 28 rule, a person born on February 29 completes the next displayed calendar year on the last day of February in a non-leap year. Thus February 29, 2000 through February 28, 2001 is shown as one completed year. The next-birthday calculation also points to February 28 until a true leap year supplies February 29 again.
March 1 is another convention
With the March 1 rule, the same February 29, 2000 through February 28, 2001 interval has not yet completed its first calendar year. It is represented as 0 years, 11 months, and 30 days under this method, and the next birthday is March 1. Showing the convention beside the result prevents two correct-but-different policies from looking like an unexplained calculator error.
Do not use a general calculator as legal advice
Age of majority, licensing, benefits, insurance, employment rules, contracts, and filing deadlines can define when a leap-day anniversary is legally reached. The applicable rule may depend on jurisdiction and purpose. Treat this tool as transparent calendar arithmetic only. For a consequential decision, read the governing statute, policy, or contract and obtain qualified advice where appropriate.
Check a leap-year cycle
A year divisible by four is usually a leap year, but century years must also be divisible by 400. That is why 2000 had February 29, while 1900 did not and 2100 will not. The next-birthday date should therefore be February 29 in 2028 for either convention, then return to the selected observed date in 2029, 2030, and 2031.
Troubleshoot a one-day disagreement
If another calculator differs by one day or one completed year, compare its non-leap anniversary policy and whether it counts the starting day inclusively. This calculator counts elapsed date boundaries, so a date compared with itself has zero elapsed days. Also confirm that the source record really says February 29 and that no time-of-day or time-zone conversion was introduced before entering the date-only values.