Age Calculator
Enter a date of birth for the exact age in years, months and days — plus the weekday you were born on and how long until the next birthday. Calculated on your device.
Enter a date of birth to see the exact age. Nothing is uploaded — the calculation runs on your device.
How to use the age calculator
- Enter the date of birth. Future dates are rejected with an explanation rather than producing a nonsense result.
- Optionally change the second date. It defaults to today, but any date works — past or future.
- Read the exact age in years, months and days, along with the weekday of birth and the countdown to the next birthday.
- Check the other units underneath if you need the same span expressed in months, weeks, days, hours or minutes.
Nothing is uploaded. The calculation runs entirely on your device.
When you would need this
Filling in a form that wants an exact age. Visa applications, insurance quotes, medical forms and school registrations often want age at a specific date rather than today, and getting it wrong can invalidate a submission.
Working out eligibility. School year cut-offs, pension ages, licence eligibility and age-restricted activities all hinge on age at a particular date rather than in a particular year.
Checking a child's age in months. Paediatric guidance, developmental milestones and nursery bandings are typically stated in months, and converting from a birth date by hand is error-prone.
Planning around a birthday. How many days until it, and what weekday it falls on.
Satisfying curiosity. How many days you have been alive, or what day of the week you were born on, are the questions this tool gets used for most.
Why calendar arithmetic is harder than it looks
Date differences seem like simple subtraction, and they are not. Three things get in the way.
Months are not a fixed length. They range from 28 to 31 days, so "one month later" means a different number of days depending on where you start. This is why age cannot be computed by dividing total days by 30.4 and rounding — that approximation drifts by several days over a lifetime.
The naive borrow produces impossible answers. The obvious algorithm subtracts day numbers and, when the result is negative, borrows the length of the previous month. That works most of the time and breaks badly at month ends. From 31 January to 1 March, it borrows February's 28 days from a difference of −30 and reports negative two days. Counting forward instead — add whole months, clamping 31 January to 28 February because there is no 31 February, then count the remaining days — gives the answer people expect: one month and one day.
Leap years have an exception to the exception. Most people know that years divisible by four are leap years. Fewer know that century years are not, unless they are also divisible by 400. So 1900 was not a leap year and 2000 was. Anyone born before 1900 whose age is computed with the simple rule will be a day out.
All of this is why the calculation here is unit tested against these specific cases rather than assumed to work.
Tips
Use the second date field for eligibility questions. "Will they be 5 by 1 September?" is answered directly by setting that date, rather than by counting forward from today.
Age in months is the right unit for infants. Developmental guidance is written in months for a reason — the difference between 6 and 9 months is enormous, while the difference between 36 and 39 is not.
Remember that age is completed years. Someone in their twenty-fifth year is 24. Legal and administrative age is always the last birthday reached, never the one approaching, which is why the day before a birthday still shows the previous age.
Check the weekday against a known date. If you are using this for genealogy or record-checking, verifying that a known date produces the expected weekday is a quick way to confirm you have entered the year correctly.
Limits and requirements
Local calendar dates only. Time of day and time zone are deliberately ignored, since a birthday is a calendar fact rather than a moment.
Proleptic Gregorian calendar. Dates are interpreted using today's calendar rules extended backwards. For dates before the Gregorian reform — 1582 in Catholic Europe, 1752 in Britain and its colonies, later elsewhere — historical records may use the Julian calendar, which differs by up to thirteen days. This matters for genealogy and for nothing else.
Four-digit years. Two-digit years are ambiguous and rejected.
29 February follows one convention. Non-leap-year anniversaries are treated as 1 March. Some jurisdictions use 28 February. Check locally if a precise legal date matters.
Browser support. Any modern browser on desktop or mobile. The date fields use your device's native date picker, so it will look and behave the way you are used to.
Privacy: how this tool handles your date of birth
The calculation runs in your browser on your own device. Nothing is transmitted, logged or stored, and closing the tab discards it.
This is worth stating plainly because a date of birth is not trivial data. It is one of the three or four details most commonly used to verify identity over the phone or online, alongside a name and an address. It appears in credit checks, account recovery flows and security questions. It is also permanent — unlike a password, you cannot change it after a breach.
Handing it to a website to satisfy a moment's curiosity is a poor trade, and an unnecessary one: this calculation is a few lines of arithmetic that any browser can do locally.
You can verify the claim rather than trusting it. Open your browser's developer tools, switch to the Network tab, and enter a date — no request carries it. Or disconnect from the internet after the page loads; the calculator keeps working.
Frequently asked questions
How is age in years, months and days actually worked out?
By counting forward, not by subtracting. The calculator finds the largest whole number of months that fit between the two dates, then counts the leftover days from there. This matters because months have different lengths — the naive approach of subtracting day numbers and borrowing from the previous month produces wrong answers, and sometimes negative day counts, whenever the birth day is later in the month than the target day.
What happens if someone was born on 29 February?
In a leap year the birthday falls on 29 February. In other years the calculator treats 1 March as the anniversary, which is the convention most legal systems use for reaching an age. Some jurisdictions and organisations use 28 February instead, so if a precise legal date matters, check the rule that applies where you are.
Why do the total days and the years-months-days figures look inconsistent?
They are answering different questions. 'Total days' counts every day that has passed. 'Years, months and days' expresses the same span in calendar units, where a year can be 365 or 366 days and a month can be 28 to 31. Both are correct — 400 days is genuinely both '400 days' and '1 year, 1 month and a few days'. The unit figures each count whole units independently rather than being remainders of each other.
Does it account for leap years?
Yes, throughout. The total-days figure counts real elapsed days including every leap day, and the month arithmetic uses each month's actual length. The leap year rule itself is handled properly too: years divisible by 100 are not leap years unless they are also divisible by 400, which is why 1900 was not a leap year and 2000 was.
Can I calculate age at a past or future date?
Yes. The second field defaults to today, but you can set any date — useful for working out how old someone will be at a wedding, what age a child will be when they start school, or how old someone was on a particular date in the past. A 'Today' button restores the default.
Is my date of birth sent anywhere?
No. The calculation runs in your browser using JavaScript on your own device. Nothing is transmitted, logged or stored. A date of birth is one of the most commonly used identity-verification details there is, so it is worth knowing which sites are collecting it — this one does not.
Does the time of day or time zone affect the result?
No, deliberately. A birthday is a calendar fact rather than an instant in time, so the calculation works on local calendar dates and ignores clock time and time zone offsets entirely. Someone born in Tokyo does not become a day older when they fly to London.
How is 'days until the next birthday' calculated?
It counts whole days from today to the next occurrence of the birth month and day. On the birthday itself it shows zero and reports the age just reached rather than the next one. For a 29 February birthday in a non-leap year it counts to 1 March, matching the convention described above.