How to Calculate Age and Days Between Two Dates

To calculate an exact age, count the full years since the birth date, then the full months since the last birthday, then the remaining days. To count the days between two dates, subtract the earlier date from the later one — the Age calculator and the Date difference calculator do both for you.

How age is calculated

Age is not "days lived ÷ 365". Years and months have different lengths, so the reliable method works in three steps:

1. Years: count the birthdays that have already happened.
2. Months: count the full months from the last birthday.
3. Days: count the days left over.

Example: someone born on 15 May 1990, on 21 September 2026.

• Their last birthday was 15 May 2026, so they are 36 years old.
• From 15 May 2026 to 15 September 2026 is 4 months.
• From 15 September to 21 September is 6 days.

Result: 36 years, 4 months and 6 days. That is 436 full months, 1,896 full weeks or 13,278 days, and their next birthday (15 May 2027) is 236 days away.

Days between two dates

Formula: number of days = later date − earlier date.

From 1 January 2024 to 1 January 2025? → 366 days, because 2024 was a leap year. The same span one year later, from 1 January 2025 to 1 January 2026, is 365 days.

Leap years

A leap year adds 29 February. The rule is:

• a year divisible by 4 is a leap year,
• except century years (1900, 2100), which are not,
• unless they are divisible by 400 (2000 was a leap year).

Leap years are the main reason why "multiply the years by 365" gives a slightly wrong total. People born on 29 February usually celebrate on 28 February or 1 March in other years; which date counts legally depends on the country.

"Include end date": the off-by-one question

By default, a date difference does not count the end date: from Monday to Wednesday is 2 days. Whether you should include it depends on what you are counting.

Example: from Monday 5 October to Friday 9 October 2026.

• End date not included: 4 days. This is right for hotel nights (you check out on Friday) or "how many days until…".
• End date included: 5 days. This is right for a leave request or an event running from Monday to Friday, where both the first and the last day are used.

If your count is always one day short or one day too many, this setting is almost always the cause.

Counting working days

Working days here means weekdays, Monday to Friday. The Date difference calculator shows them next to the total, but public holidays are not excluded, because they differ by country and region. Subtract them manually.

Example: December 2026, from 1 to 31 December with the end date included.

• Weekdays: 23.
• Christmas Day (25 December) falls on a Friday: 23 − 1 = 22.
• In the UK, Boxing Day falls on a Saturday in 2026, so the substitute bank holiday is Monday 28 December: 21 working days.

Also remove any days your workplace is closed, such as company shutdowns.

Quick method with our tools

1. Open the Age calculator and enter the date of birth.
2. Leave "Age on" as today, or choose another date (for example, a deadline for an application).
3. Read the age in years, months and days, plus the totals and the days until the next birthday.
4. For any other span, open the Date difference calculator, enter the two dates and choose whether to include the end date.
5. Read the days, weeks, months and weekdays, then subtract public holidays yourself.

The same calculator can also add or subtract days from a date, for example to find the date 90 days from today.

Common mistakes

• Dividing days by 365 to get an age: leap years make the result drift, especially near a birthday.
• Forgetting the end date in leave or event counts, or adding it to hotel nights.
• Treating weekdays as working days without removing public holidays.
• Mixing date formats: 03/04 is 3 April in most of Europe but 4 March in the US. Check which one a form expects before you type a date.