Age Calculator

What this tool does

Age Calculator finds the exact years, months and days between a date of birth and any target date (defaulting to today), using the same borrow-as-you-go method as working it out by hand on a calendar. It also reports total days lived, total weeks, total months, and how many days remain until the next birthday.

A birthday tells you how many full years old someone is, but it hides the months and days on top of that, and figuring out an exact age, or the exact gap between any two dates, by hand means carefully borrowing days and months when a straight subtraction would go negative — the same tricky bit of arithmetic behind every by-hand age calculation.

This calculator takes a date of birth and a target date, defaulting to today, and works out the exact years, months and days between them using that same borrowing method, so the result matches what you would get counting carefully on a calendar. It also reports the total days lived, total weeks, and total months, plus a countdown to the next birthday, including the correct handling of a 29 February birthday in non-leap years.

How to use Age Calculator

1. Enter the Date of birth.
2. Enter the As of date, or leave it on today for current age.
3. Read the Age result as years, months and days.
4. Check Total days lived, Total weeks and Total months for alternative ways to express the same span.
5. Check Days until next birthday and the date it falls on.

Tips for better results

• Someone born on 29 February has their "legal" birthday handled differently in different places; this calculator treats 28 February as the practical fallback in non-leap years when counting down to the next birthday.
• Total months rounded down is not the same as years × 12 + months of the breakdown unless the day-of-month components line up exactly; both figures are shown so you can see the difference.
• Setting the As of date to a past date works just as well as today, letting you find someone’s exact age on any specific historical date.
• The borrow-based breakdown (years, months, days) is the one that matches how people actually describe age in everyday speech, unlike a flat total-days-divided-by-365 estimate.

Common use cases

• Finding someone’s exact current age in years, months and days for a form, certificate or record.
• Working out how many days old a baby or pet is.
• Checking a person’s exact age on a specific past or future date, such as an eligibility cutoff.
• Counting down the days remaining to an upcoming birthday.

Common mistakes to avoid

• Assuming age in years alone captures the full picture; two people both "30 years old" could be up to almost 365 days apart in actual age.
• Using a flat 365-day-year estimate instead of exact calendar arithmetic, which drifts increasingly from the real answer across leap years.
• Forgetting that a 29 February birthday needs a defined fallback date in non-leap years when working out a birthday countdown.