Age Calculator

Why age arithmetic is harder than it looks

Calculating an age seems like it should be subtraction, and it is one of the classic sources of bugs in software. Months have different lengths, so 'one month after 31 January' has no obvious answer. Years have different lengths, so dividing total days by 365 slowly drifts and will eventually report your age wrongly. Leap years arrive every four years, except in century years, except in century years divisible by 400 — which is why 1900 was not a leap year and 2000 was. Time zones mean two people can disagree about what day it currently is. The reliable approach is the one humans use intuitively: count whole years, then whole months, then the days left over, working in calendar terms rather than trying to reduce everything to a number of days first.

Where an exact age actually matters

Precision beyond 'about forty' matters more often than you would think. Visa and immigration forms frequently require an age in years and months on a specific date. Insurance premiums and pension calculations can step at exact ages, and being one day either side of a threshold changes the number. School enrolment cut-offs are defined to the day. Paediatric medicine dosing is calculated by exact age in months for infants, where a few weeks is a large proportion of a life. Legal thresholds for consent, employment and liability are all defined precisely. In each case an approximation is not merely imprecise — it is the wrong answer to a question that has a right one.