Calculating age sounds simple — subtract the birth year from the current year — but that alone is often wrong, since it ignores whether the birthday has actually happened yet this year. This calculator instead works out the full year/month/day difference between two exact calendar dates, borrowing from the previous month the same way you'd borrow in subtraction, which correctly handles month-length differences and leap years without any manual adjustment.
This exact-date approach is also why "total days lived" is genuinely useful beyond curiosity — it's the same calculation used for things like eligibility windows, insurance age-banding, and precise legal age requirements that specify an exact date rather than just a birth year.
Automatically and correctly — since the calculation works from actual calendar dates rather than an average of 365.25 days per year, leap years (and the extra day they add) are already accounted for in the year/month/day and total-days results.
The most common cause is a different definition of "years old" — some tools round to the nearest year, others always round down (the standard, and what this calculator does), and a few use a simplified 30-days-per-month approximation instead of real calendar dates, which can be off by a day or two around month boundaries.
Yes — use the "calculate as of a different date" option to set any custom starting date instead of your date of birth, which turns this into a general date-difference calculator.
In non-leap years, this calculator treats your birthday as having occurred once February ends (the standard convention), so your "days until next birthday" figure will still be accurate even in years without a February 29th.