Age Calculator

Enter a date of birth and this returns an exact age: whole years, months and days, plus running totals in weeks, hours, minutes and seconds. It also shows how long until the next birthday, which age that will be, the star sign, and which day of the week the birth fell on.

How to use it

1. Enter the date of birth. The field will not accept a date in the future.
2. The main figure is the age in completed years. Underneath, the exact breakdown in years, months and days appears.
3. Scroll the totals to see days lived, weeks, hours and minutes, with a live seconds counter.
4. The birthday line tells you how many days remain and which age is next.
5. To measure to a date other than today, fill in the second field. That turns it into a general date-difference calculator.

Why calendar age is not a division

The obvious way to calculate an age is to subtract two dates, divide the days by 365.25, and take the whole number. That gets the years roughly right and everything else wrong, and it is why so many age calculators disagree with each other by a day.

The problem is that months have different lengths and years are not all the same size. Consider someone born on 31 March, and today is 30 April. Have they completed a month? By any sensible reading, not quite — but if you assume months are 30 days long, the arithmetic says one month and zero days.

Now take a birth date of 31 January measured on 1 March in a common year. February had 28 days. Naive subtraction gives a negative day count, which then has to be corrected by borrowing from somewhere. Borrow a fixed 30 days and you get “1 month and -2 days” turning into odd results; borrow the actual length of February and you get one month and one day, which is right.

This calculator borrows from the real preceding month every time. It walks the years first, then the months, then the leftover days, and when the day count goes negative it takes exactly as many days as the previous calendar month contained. That is why the days figure here is always between zero and thirty, never negative, and never larger than the month it came from.

Leap years create a related puzzle with no purely mathematical answer. Someone born on 29 February has an anniversary only every four years. Legally, England and Wales treat their birthday in a common year as 1 March, which is the rule used here. Other jurisdictions use 28 February. Neither is more correct; the difference matters mainly for people turning 18 or 21, and it is worth knowing that a tool has to pick one.

What it does not do

It works in whole calendar days and uses UTC internally, so it does not account for the timezone or the hour of birth — an age in hours is therefore accurate to within a day. It has no opinion on the “traditional East Asian age” systems, which count differently again. The star sign uses standard Western tropical dates, which shift by roughly a day between years; if a birth date falls on a cusp, treat the answer as indicative.

Nothing you enter leaves your browser. The date is never uploaded, and the shareable link encodes it in the URL rather than storing it anywhere.

Days in each month

Month	Days	Leap year
January	31	31
February	28	29
March	31	31
April	30	30
May	31	31
June	30	30
July	31	31
August	31	31
September	30	30
October	31	31
November	30	30
December	31	31

Star sign dates

Sign	Dates
Capricorn	22 Dec – 19 Jan
Aquarius	20 Jan – 18 Feb
Pisces	19 Feb – 20 Mar
Aries	21 Mar – 19 Apr
Taurus	20 Apr – 20 May
Gemini	21 May – 20 Jun
Cancer	21 Jun – 22 Jul
Leo	23 Jul – 22 Aug
Virgo	23 Aug – 22 Sep
Libra	23 Sep – 22 Oct
Scorpio	23 Oct – 21 Nov
Sagittarius	22 Nov – 21 Dec