Illustrative example. The dates and amounts below are provided for practice.
Someone born February 29, 2020 is being described on February 28, 2025. The completed-year figure depends on which common-year observance is selected.
Change only the policy
Enter those two dates. With February 28 in common years selected, the Years result is 5. Switch to March 1 in common years and calculate again. Years becomes 4, and the next birthday is March 1, 2025.
The elapsed total remains 1,826 days in both runs. The rule determines the calendar anniversary anchor. It does not change the number of actual date intervals since birth.
Pick a rule for the purpose
For a personal celebration, use the person's preferred date and save the same rule on their tracker record. For an official age question, check the rules that apply to the particular form or institution.
An inclusive count beginning on the birth date would call February 28, 2025 Day 1,827. That convention does not resolve which date counts as the fifth anniversary.
If a caption contains both a completed age and total days, keep the selected rule with the saved result. Otherwise, a future reader may see different year counts without knowing why.