The rule has an exception to an exception
A Gregorian year divisible by four is a leap year, except for century years that are not also divisible by 400. That makes 2000 a leap year, while 1900 and 2100 are not. The U.S. Naval Observatory explains the rule and its calendar background in the reference below.
Check the exact dates in a span
An interval that crosses February does not always contain an extra day. February 28 to March 1 is two elapsed days in 2024 and one in 2025. Let a date calculation use the actual dates rather than adding a leap-day adjustment to an estimate.
Separate the date from the celebration
The calendar date of February 29 is absent in most years. A birthday countdown therefore needs an explicit convention for those years. AgeUse uses February 28. This describes the tool's arithmetic; a personal celebration can be planned differently.