How do I calculate my age in months?

Count the complete months from the birth date. Someone aged 36 years and 3 months is 435 months old.

How do I find my age on a past or future date?

Use that date instead of today when you count the years, months and days.

When is the birthday of someone born on 29 February?

Every 4 years on the actual date, and in other years on 28 February or 1 March depending on local convention.

Dates and time

How to Calculate Age From a Date of Birth

Age is a count of complete years, then leftover months and days. Here is the method and the leap-day edge case.

By Editorial Team · Updated September 19, 2026 · 2 min read

Your age is the number of complete years since you were born. You can also express it as years, months and days, or as a total number of days.

Years, months and days

Start with the year difference between the two dates.
If the birthday has not yet happened this year, subtract one year.
Count the whole months since the last birthday.
The leftover days are the days since the most recent monthly anniversary.

Example: born 15 June 1990, checked on 19 September 2026.

The birthday for 2026 has passed, so the age is 2026 − 1990 = 36 years.
From 15 June to 15 September is 3 months.
From 15 September to 19 September is 4 days.

The age is 36 years, 3 months and 4 days.

Total days

The number of days lived is the gap between the two dates. For this example it is 13,245 days, which is about 435 months.

Days until the next birthday

Find the next occurrence of the birthday and count the days to it. The next birthday is 15 June 2027, which is 269 days from 19 September 2026, and it falls on a Tuesday.

Leap-day birthdays

People born on 29 February have a real birthday only in leap years. In other years they mark it on 28 February or 1 March, depending on the country and the purpose. The convention is set by local law for things like driving or voting age, so check the rule that applies to you.

Month-end birthdays

If you were born on the 31st and the current month has 30 days, the monthly anniversary falls on the last day of that month. Good tools clamp to the end of the month instead of skipping it.

Common mistakes

Subtracting years only, without checking whether the birthday has passed.
Assuming every month has 30 days when counting leftover days.
Ignoring time zones when a birth or comparison date is near midnight.

The age calculator shows all of this at once, and lets you check age on any date, not just today.

Try it yourself

Born 3 August 2001. How old is that person on 19 September 2026?
Born 15 June 1990. How old on 14 June 2026?
Someone is 36 years and 3 months old. How many months is that in total?

Answers: (1) 25 years, 1 month and 16 days; (2) 35 years, 11 months and 30 days, one day short of turning 36; (3) 435 months.

Frequently asked questions

How do I calculate my age in months?: Count the complete months from the birth date. Someone aged 36 years and 3 months is 435 months old.
How do I find my age on a past or future date?: Use that date instead of today when you count the years, months and days.
When is the birthday of someone born on 29 February?: Every 4 years on the actual date, and in other years on 28 February or 1 March depending on local convention.

About the author

Editorial Team. The Editorial Team writes, checks and maintains every tool and guide on this site. The calculation logic behind the calculators is covered by automated tests, examples are worked out and re-checked before publishing, and pages are updated when we find a mistake or a rule changes. Corrections are welcome through the contact page.

How we write and check our content