Age & Date Difference Calculator

Calculate your exact age or the days between two dates

Need to know your precise age, how many days until your birthday, or how much time passed between two events? This free age calculator and date difference calculator gives you accurate results in years, months and days — plus total days, weeks and more — without installing an app or creating an account.

How the age calculator works

Enter your date of birth and an as-of date (today by default). The calculator counts full years, then remaining months, then remaining days — the same way you'd figure age on paper, but instantly. It also shows total days lived, your next birthday, and days until that birthday.

How the date difference calculator works

Switch to Date difference, pick a start date and an end date, and see the span between them in years, months and days, along with the total number of days and weeks. Useful for project timelines, rental periods, pregnancy tracking, legal deadlines and everyday questions like "how many days since…?"

Leap years and calendar accuracy

The calculator uses real calendar dates, so it correctly handles months with different lengths and leap years. February 29 birthdays and cross-month calculations are all accounted for.

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How age and date differences are calculated

Human age is not "total days ÷ 365" — months have different lengths and birthdays do not align neatly with calendar years. This tool counts the way people actually state age: full years first, then leftover months, then leftover days.

Age mode (date of birth → as-of date)

Starting from the birth date, the calculator adds full years until the next birthday would pass the as-of date. It then counts whole months from that anniversary, and finally the remaining days. If the as-of day is before the birth day-of-month in the current month, it borrows one month and adds that month's day-count to the day remainder — the same carry you would do by hand.

Age = complete years + complete months + remaining days
Total days lived = (as-of date − birth date) in calendar days

Date difference mode

The same year-month-day counter runs between any two dates. Total days is the inclusive-friendly count of calendar days spanned (end − start in days). Weeks are total days ÷ 7 — useful for project planning but not the same as "calendar weeks on a wall chart" if you need ISO week numbers.

Worked examples

Common mistakes to watch for

• Mixing up "age in years" with "total days ÷ 365.25". A person aged 30 has lived about 10,957 days, but dividing days by 365.25 does not recover clean years/months because of leap years and month boundaries.
• Off-by-one day confusion. Legal deadlines ("within 30 days of signing") sometimes count inclusively, sometimes exclusively. This tool reports the calendar span between two dates; check your contract's definition if the last day itself matters.
• Timezone shifts on birth timestamps. For a birth at 11:30 pm in one timezone, the calendar date can differ from UTC. Enter the local calendar date on the birth certificate, not a converted instant from a hospital system unless you know the intended date.
• Using date difference for "how old will I be on…". That is Age mode with a future as-of date. Date difference compares two arbitrary events — project start/end, warranty periods — without treating either as a birth date.

When to use each tab

Use Age when one date is a date of birth (or incorporation date, domain registration — any fixed start anchor) and you want human-readable age plus total days lived and next birthday countdown. Use Date difference when neither date is special — rental length, employment gap, days until a deadline, time between two historical events.

For clock times across regions ("3 pm in London is what in Tokyo?"), use the Time Zone Converter. For body-weight tracking over time, the BMI Calculator handles height/weight — not calendar spans.

Because everything runs on your device, there's nothing to install and nothing is uploaded. Your dates stay completely private, and the calculator works even when you're offline. This is one of many free browser tools on Uzetool — explore more tools as we add them.