Find exact age in years, months and days from any birth date.
Age looks trivial and is not. The common method counts completed years since birth, so you remain a given age until the day before your next birthday. Calculating it properly means comparing year, month and day separately: subtract the birth year from the current year, then reduce by one if the current month and day fall before the birthday. Simply subtracting years is wrong for anyone whose birthday has not yet occurred this year.
Two edge cases cause genuine disagreement between systems. People born on 29 February have no birthday in most years, and different jurisdictions and software resolve this differently — some treat 28 February as the anniversary, others 1 March. And several East Asian age-reckoning traditions count a person as one at birth and add a year at new year rather than on the birthday, which can put someone one or two years older than the international reckoning. If an age matters legally or medically, it is worth knowing which convention is in use.
By completed years, months and days. Subtract the birth year from the current year, then subtract one if the current date falls before the birthday this year. The remaining months and days are counted from the last birthday.
It depends on the system. In non-leap years some treat 28 February as the anniversary and others 1 March, and legal definitions vary by jurisdiction. The person still ages one year; only the nominal date shifts.
Usually a time-zone difference, or a different convention for counting months and partial days. Some tools count completed months only, others include partial ones, which shifts the result by up to a day.
Several traditional systems count a newborn as one year old and add a year at the lunar or calendar new year rather than on the birthday. This can make someone one or two years older than the international count.