Age calculation seems trivial until leap years enter the picture — and a surprising number of hand-built spreadsheets and simple scripts get it subtly wrong around February 29.
Why leap years complicate age math
A correct age calculation needs to compare not just the year difference, but whether the current month and day have "occurred yet" relative to the birth month and day this year — and for someone born on February 29 specifically, most years don't even have that exact date to compare against, since it only exists once every four years (with the century-year exception rule making it even less frequent in three out of every four centuries).
The common bug in simple age calculations
A naive approach — just subtracting birth year from current year — ignores whether the birthday has actually happened yet this year, which produces an age that's off by one for a large portion of the calendar year (anyone whose birthday hasn't occurred yet in the current year). This isn't a leap-year-specific bug, but it's the same category of "off by one" mistake that leap-year edge cases compound further.
How a correct calculation handles it
A properly built date-based calculation compares full dates, not just years, and has explicit handling for the February 29 edge case — typically treating a leap-year birthday as occurring on either February 28 or March 1 in non-leap years, depending on the convention chosen, since there's no universal legal standard for which one is "correct."
Use a tool that handles this correctly
An age calculator built on proper date arithmetic accounts for both the "hasn't happened yet this year" issue and the leap-year edge case, rather than the naive year-subtraction approach that produces an incorrect result for a meaningful share of birthdates.
The bottom line
Age math looks simple but has real edge cases — leap-year birthdays being the most notorious — that a properly built calculator handles correctly and a quick manual subtraction usually doesn't.
Try it yourself
Put this into practice with our age calculator.