How this instrument works
"How old are you?" has a lazier answer and an exact one. The exact one counts whole calendar years, then whole calendar months, then leftover days — respecting the different lengths of months and every leap-year rule along the way. That is genuinely fiddly arithmetic: subtracting 1990-05-20 from 2026-07-18 is not one subtraction but a cascade of borrowings against a calendar where February keeps changing size.
This instrument does the cascade on the civil calendar — the same Gregorian rules your passport office uses — and also reports the blunt totals: days lived and full weeks. The total-days figure is computed independently of the years-months-days breakdown, which is exactly how the two are kept honest against each other in testing.
- Enter the date of birth.
- The second field defaults to today — change it to compute an age on any date, past or future.
- Read the age breakdown, the day total, and the countdown to the next birthday.
Worked example — born 20 May 1990
On 18 July 2026, someone born 20 May 1990 is 36 years, 1 month and 28 days old: 36 full years to 20 May 2026, one full month to 20 June, then 28 days to 18 July.
The independent day-count agrees: 36 years spanning nine leap days is 13,149 days, plus the 59 days from 20 May to 18 July, totalling 13,208 days — about 1,886 weeks of being alive. Two computations, two routes, one answer; that is the house method.
Questions
I was born on 29 February — when is my birthday?
This instrument observes leap-day birthdays on 1 March in common years, which matches most legal conventions (and how the countdown behaves here). Some jurisdictions use 28 February instead — if yours does, your legal age flips a day earlier three years out of four.
Why does the months part of my age jump around?
Because months are different lengths. Turning '1 month and 3 days older' is 31 days across July but 28 across February. The years-months-days form is calendar-true rather than arithmetically smooth — that is a feature; the totalDays figure is the smooth one.
Can I compute an age on a past or future date?
Yes — set the second field to any date. School-entry cutoffs, retirement dates, 'how old was she when…' questions: it is the same subtraction pointed at a different day. The instrument only refuses dates before the birth date.
Does this handle time zones or the time of day?
Deliberately not. Legal age changes at midnight on the calendar date, wherever you are; adding clock time would add precision the question does not actually have. Dates in, dates out.