Age Calculator

Enter a date of birth and a comparison date, and this returns age at last birthday - completed years, plus the whole calendar months and leftover days since that birthday - along with total months, total weeks, exact elapsed days, the date of the next birthday and how many days remain until it. CDC/NCHS defines age at last birthday as age in completed years, calculated from a date of birth and a reference date, and that is the definition used here. The mechanism is worth understanding because it is not what most people assume. Age is not elapsed days divided by 365.25. It is a calendar comparison: take the difference in years, then subtract one if this year's birthday has not yet arrived. That is the whole of the year calculation. The months and days are then measured by advancing actual calendar months forward from your last attained birthday until the next step would overshoot the comparison date, and counting the remaining days after that. Dividing days by an average year or month instead produces answers that are subtly wrong and occasionally embarrassingly wrong. An average month has no fixed length - February has 28 or 29 days, and seven months have 31 - so "3 months and 12 days" computed by division does not correspond to any real span of calendar dates. Worse, near a birthday the accumulated drift from a 365.25-day year can flip the headline year figure, which is the number people actually care about. Measuring from the last birthday in real calendar months means the components always describe a span you could point to on a wall calendar, and short months never distort the result. Total days is computed separately as an exact count of midnights crossed between the two dates, so nothing about it depends on the month decomposition. Then there is 29 February, which forces a decision no amount of arithmetic can avoid: in a common year, does a leap-day birthday fall on 28 February or 1 March? Different jurisdictions answer differently, so this page states its rule rather than hiding it. It treats 1 March as the anniversary in non-leap years, the convention set out in section 6 of the Age of Legal Capacity (Scotland) Act 1991. That choice has a visible consequence: a leap-day baby turns another year older on 1 March, not 28 February, so on 28 February in a common year they are still one year younger than a friend born a day earlier. The month-stepping loop is capped at eleven for precisely this reason - adding twelve months to 29 February clamps to 28 February in a common year, but the declared anniversary has not yet arrived, so the completed-years calculation owns that boundary and the loop only decomposes what is left over. This is popular usage rather than a legal-age determination. Federal regulation at 42 CFR 436.522 distinguishes popular usage, where an age is attained on the birthday anniversary, from the older common-law rule under which a person attains an age on the day before. Use it for everyday purposes, not for establishing eligibility. The comparison date is an explicit input rather than a reading of the system clock, so a shared result link gives the same answer forever instead of quietly drifting. Birth dates after the comparison date are refused rather than reported as negative. For a plain span between two arbitrary dates, including business-day counts, use the date calculator; if you are working out a retirement horizon from the age this page returns, the retirement calculator takes it from there.