Illustrative example. The dates and amounts below are provided for practice.
A commemorative record spans 1900, a year that is divisible by four but is not a leap year. The century exception matters when checking the day total.
Enter Date of Birth February 28, 1896 and Calculate Age As Of March 1, 1900. The result is four years and one day, with 1,462 elapsed days. There is a leap day in 1896 after the starting date, but no February 29 in 1900.
Inspect the boundary rather than guessing
Trying to enter February 29, 1900 as a birth date should fail validation. The Gregorian rule excludes century years unless they are divisible by 400. The year 2000 qualifies, while 1900 does not.
Inclusive numbering beginning on February 28, 1896 would call the reference date Day 1,463. The extra one comes from counting the first date, not from another leap day.
Use records carefully
This calculation is suitable when both dates are understood as Gregorian dates. If a historical document used a different calendar, establish its conversion separately before entering it. An apparently one-day disagreement can come from the source date rather than the arithmetic.