Age Calculator

Enter a birth date and an “as of” date. Everything is calculated on your device — the dates are never uploaded.

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How the age is counted

Ages feel simple until you try to write the rule down. This tool counts the whole calendar months between the two dates first — the difference in years times twelve, plus the difference in months — and then walks back one month if that anchor lands after the target date. The days that remain are measured against that anchor. It is the same approach mature date libraries use, and it means every intermediate month contributes its real length rather than an assumed 30 or 31 days.

The awkward cases

  • Month-end starts. 31 January towards 1 March has no "31st" in February, so the anchor clamps to 28 February (2023) and one day is left over: one month and one day, not two months.
  • Leap days. Someone born on 29 February only has a "real" birthday every four years. The countdown here shows 1 March in common years so it never appears to have already passed.
  • Total days vs. total months. These do not divide neatly. Ninety days after a date might be two months and twenty-nine days, because the months it crossed were short ones.

What you can do with the totals

The total-days figure is handy for "days since" milestones — a 10,000-day anniversary, a project start date, a sobriety count. Total weeks maps cleanly onto pregnancy weeks or sprint counts. If you are working the other way and need a share of a number rather than a span of time, the percentage calculatorcovers that. For a quick random draw — a birthday raffle, say — theUUID generator and theword counter sit alongside this one in the toolbox, all running entirely in the page with nothing sent anywhere.

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Frequently asked questions

How is the age worked out?
It counts whole calendar months between the two dates, then the leftover days against that month anchor. So 1990-06-15 to 2023-08-20 is 33 years, 2 months and 5 days — the same result a library like dateutil's relativedelta gives.
What does the “as of” date do?
It is the date you are measuring the age at. It defaults to today, but set it to a future date to see how old someone will be then, or a past date to check an age on a particular day.
How are month-end dates handled?
When the start day does not exist in the end month — 31 January towards 1 March, say — the month is clamped to that month's real length (28 or 29 for February) before the leftover days are counted. 2023-01-31 to 2023-03-01 comes out as 1 month and 1 day.
What happens with a 29 February birthday?
In a common year the next birthday is shown as 1 March, so the countdown never lands before the true anniversary. In a leap year it stays on 29 February.
Why do total months and years-plus-months differ from total days ÷ 30?
Months are not all 30 days. Total days is the exact count between the dates; total months is how many whole calendar months fit, which is why 90 days can be 2 months and 29 days rather than 3.
Is my birth date stored or sent anywhere?
No. The calculation is pure JavaScript in your browser. The dates are put in the page URL only if you use “Copy link to this result”, so you can share a specific calculation — nothing is logged server-side.
Which weekday was I born on?
The tool shows it from the birth date directly. It uses the proleptic Gregorian calendar, so dates before 1582 will not match historical records that used the Julian calendar.

Last reviewed: September 2026. Figures and formulas are checked against their published sources; see the site's data notes.