Why age calculation is trickier than it looks
Subtracting years is easy, but months and days are irregular. Months have 28 to 31 days, and leap years add a 29 February every four years, except century years not divisible by 400. A naive calculator that divides total days by 30 or 365 drifts by days over a lifetime.
This calculator counts whole calendar months from the date of birth. It finds the most recent monthly anniversary on or before the calculation date, then counts the remaining days. When an anniversary would fall on a day that does not exist, such as 31 April, it uses the last day of that month. So someone born on 31 January is exactly one month old on 28 February in a common year, and one month and one day old on 1 March.
Leap-day birthdays
If you were born on 29 February, the next birthday shown in common years is 28 February, which is the convention used by many jurisdictions for legal age. Some places use 1 March instead; check local rules if the exact day matters legally. All calculations use dates without time zones, so the result does not shift with daylight saving time.