How to Calculate Age
By Tools Star Hub Editorial Team · Published · Updated
Age on this page is the civil-calendar gap from a date of birth to a later date, counted in years, months, and days, plus a total in days. Someone born on 1 January 2000 is 26 years, 0 months, and 0 days on 1 January 2026, which is 9,497 days. A birth date of 29 February 2000 reaches 28 February 2001 as 0 years, 11 months, and 30 days, which is 365 days. The selected date cannot be before the date of birth.
Subtracting the years and ignoring the month is wrong when the birthday has not arrived. Counting days with a fixed 365 misses leap days. This calculator walks the calendar, so a leap day is a real day in the total and a real complication in the year-month-day display. It does not look up a birthday or a time zone.
How are years, months, and days counted?
The count starts at the birth date and moves forward to the selected date. Full years are completed birthdays. Full months are completed month anniversaries after that. The leftover is days. On a birthday, the months and days are 0, as in the 1 January 2000 to 1 January 2026 example.
Total days is the number of midnights between the two dates, not an average year times the age. The 26-year example is 9,497 days. That includes the leap days that fell in between. The page does not show hours or minutes.
What happens with a leap-day birthday?
29 February 2000 is a real date. The next year, 2001, has no 29 February. Measured on 28 February 2001, the result is 0 years, 11 months, and 30 days, and the total is 365 days. The birthday anniversary in a non-leap year is not invented as 29 February.
An impossible date, such as 31 February, is not accepted. The date has to be a real calendar day. Use the date fields rather than a free-typed month name.
What dates are rejected?
If the selected date is earlier than the birth date, the page says The selected date cannot be before the date of birth. Swap the dates if you meant the other direction. A future date is allowed when you want an age on a later day, such as a 20th birthday that has not happened yet.
The count is date to date. It does not apply a time of day, a time zone, or a legal rule about when a birthday is observed. For the number of days between two dates that are not an age, the date-difference calculator is the other tool.
The day before that anniversary is not yet 26. On 31 December 2025, a 1 January 2000 birth is 25 years, 11 months, and 30 days, which is 9,496 days. The next day, 1 January 2026, is the first date that reads 26 years, 0 months, and 0 days.
How to calculate age
- Enter the date of birth. Use a real calendar day. 29 February is accepted only in a leap year.
- Enter the date to measure. It can be today or a later date. It cannot be earlier than the birth date.
- Read years, months, days, and total days. A birthday anniversary shows 0 months and 0 days. Total days includes leap days.
- Do not treat the total as 365 times the years. 9,497 days for 26 years is the calendar count, not 26 times 365.
Example
1 January 2000 to 1 January 2026
The age is 26 years, 0 months, and 0 days. The total is 9,497 days.
29 February 2000 to 28 February 2001
The age is 0 years, 11 months, and 30 days. The total is 365 days.
Common mistakes
Year subtraction skips the birthday
The calendar count stays at the previous age until the anniversary. Months and days show how far along the current year is.
A leap day does not exist in every year
The 2000-02-29 example does not land on 29 February 2001, because that day is not on the calendar.
FAQ
- How old is someone born on 1 January 2000 on 1 January 2026?
- 26 years, 0 months, and 0 days. The total is 9,497 days.
- How is 29 February 2000 counted on 28 February 2001?
- As 0 years, 11 months, and 30 days, which is 365 days in total.
- Can the second date be in the future?
- Yes. It cannot be before the date of birth. The page rejects that order.
- Does the calculator include the time of day?
- No. It compares calendar dates and reports years, months, days, and total days.
- Is a birthday looked up online?
- No. Both dates are the ones you enter, and the count runs in the browser.
To count age in years, months, days, and total days, use the Age Calculator. It follows the civil calendar, including leap days.
For a day count that is not an age, see How to count days between dates.
Percents are a different kind of figure. See How to calculate a percentage.