Quick answer: Age is the number of full years, months and days between a birth date and a chosen date. Someone born on 15 March 1990 is 36 years, 6 months and 4 days old on 19 September 2026. Leap days are handled by the calendar: a year is a leap year if it divides by 4, except centuries that do not divide by 400.
Age looks simple until you need it exactly: for a form, a legal cut-off, a school entry date or just curiosity. Counting whole years is easy, but adding the extra months and days by hand means dealing with months of different lengths and leap years.
This calculator does the calendar maths for you. Besides your age in years, months and days, it shows the totals in months, weeks, days and hours, the weekday you were born on, and the date and countdown for your next birthday. Change the “age at” date to work out how old someone was or will be on any day.
How the age is counted
The calculator counts complete years first, then complete months, then the remaining days, using the real calendar. A person born on January 31 who reaches February 28 has not yet completed a month, so the days count on from January 31.
Total days are the exact number of calendar days between the two dates. Total hours multiply that by 24 and ignore daylight saving changes, so treat it as approximate. Leap years are handled automatically: if you were born on February 29, your birthday is counted on March 1 in years that are not leap years.
A worked example
Example. Someone born on March 15, 1990 is, on September 19, 2026, 36 years, 6 months and 4 days old. That is 438 months, 1,905 weeks and 13,337 days in total.
Real-world example
A child born on 15 March 1990 is 36 years, 6 months and 4 days old on 19 September 2026. The tool counts complete years first (36, since the birthday in March has passed), then the complete months since the last birthday (6, up to 15 September), then the remaining 4 days. Changing the “as of” date lets you find the age on any past or future day, for example on the first day of school or a retirement date.
How the calendar counts months
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 (29 in a leap year) | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
A year is a leap year if it divides by 4, except centuries, which must divide by 400. So 2000 and 2024 were leap years, and 1900 and 2100 are not. This is why counting age in months and days follows the calendar and not a fixed 30 days.
Common mistakes to avoid
- Subtracting years only. Whether someone has already had their birthday this year changes the age by one.
- Forgetting leap days. People born on 29 February have birthdays in non-leap years on 28 February or 1 March, depending on local rules.
- Using the wrong date format. Enter dates with the date picker to avoid mixing day-month and month-day order.
Frequently asked questions
How do I calculate my age from my birth date?
Enter your date of birth and leave the second date as today. The result shows years, months and days. It updates each day if you return to the page.
Can I find out how old someone was on a past date?
Yes. Change the “age at this date” field to any date after the birth date, in the past or future.
How does the calculator handle leap years?
It uses the real calendar, including February 29. For people born on February 29, the birthday is counted on March 1 in non-leap years.
Is my date of birth saved?
No. The calculation is done in your browser and nothing is stored or sent.
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Sources and further reading
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