About the Age Calculator
This age calculator takes a date of birth and a reference date and returns the exact age in years, months and days, alongside the total days lived, total weeks and total months. It also counts the days remaining until the next birthday and tells you which age that birthday will bring. The reference date is a field rather than a hidden read of the clock, which means you can work out somebody's age at a past event or on a future date just as easily as today, and the result never changes underneath you.
How to use the Age Calculator
- 1
Enter the Date of birth using the date field.
- 2
Set Age as of to today's date for the current age, or any other date to see the age at that moment.
- 3
Read the Age panel for years, months and days plus the total days, weeks and months lived.
- 4
Check the Next birthday note for the countdown in days and the age that birthday will bring.
What people use it for
Checking eligibility on a specific date
Pension access, driving licences and school entry all hinge on an exact age on a given day, which the as-of field answers directly rather than approximately.
Filling in a form that wants years and months
Medical and childcare forms frequently ask for age in years and months, a figure that is surprisingly fiddly to work out correctly in your head.
Planning a milestone
Total days lived tells you when a 10,000-day or 20,000-day mark falls, which makes for a more distinctive celebration than a round birthday.
Age at a historic event
Set the as-of date to a past date to find out how old somebody was when something happened, useful for genealogy and biographical writing.
How age is counted, and why the convention is not universal
In most of the world age increases on the birthday itself and stays put the rest of the year, so someone born on 3 June turns 30 on 3 June and is 29 until that moment. This calculator follows that convention: if the reference date falls before this year's birthday, it reports last year's age plus the months and days that have elapsed since. The convention is not the only one in use. Traditional East Asian age reckoning counted a newborn as one year old and added a year at the lunar new year rather than on the birthday, so a person could be two by Western reckoning and four by the traditional count. South Korea used a version of this system in daily life until June 2023, when legislation standardised on the international method. Insurance underwriting has its own quirk, the age nearest birthday, which rounds to whichever birthday is closer and can make someone underwriting-older than their actual age for half the year.
The 29 February problem
Roughly one person in 1,461 is born on 29 February, and in the three years out of four with no such date their birthday has no exact anniversary. Jurisdictions differ on which day it moves to. In England and Wales the Family Law Reform Act treats an eighteenth birthday for someone born on 29 February as falling on 1 March, while New Zealand and Hong Kong use 28 February, and Taiwan uses 28 February for legal purposes as well. This calculator constructs the next birthday from the birth month and day, so JavaScript's date handling rolls 29 February forward to 1 March in a common year. The years figure is unaffected either way, because the years, months and days calculation works from real calendar positions rather than from a constructed anniversary date. If a legal deadline turns on the exact day, check the rule for the relevant jurisdiction rather than trusting any calculator's default.
Tips
- Set the as-of date deliberately — the tool never reads the clock, so it will not silently update tomorrow.
- Total days lived counts every leap day in the range, so it is exact rather than approximated from a yearly average.
- For someone born on 29 February, confirm which convention applies before relying on the next-birthday date.