How to Calculate Age and the Days Between Two Dates
Exact age and the number of days between dates come up in forms, eligibility rules, contracts and countdowns. Here is how the math works.
Age in years
Subtract the birth year from the current year, then subtract one if the birthday has not happened yet this year. Someone born on June 15, 1995 is 31 on June 15, 2026, but still 30 on June 14, 2026.
Age in years, months and days
Subtract in steps. First subtract the years, then the months, then the days. If the day subtraction is negative, borrow the number of days in the previous month and reduce the months by one. If the month subtraction becomes negative, borrow 12 months and reduce the years by one.
For example, from June 15, 1995 to October 7, 2026: years 31, months 3, days 22. That gives 31 years, 3 months and 22 days.
Counting days between two dates
The simplest method is to count the days from the start date up to, but not including, the end date. From January 1, 2026 to December 25, 2026 there are 358 days. To count both the start and the end date, add 1.
Leap years
A leap year adds February 29. Years divisible by 4 are leap years, except century years not divisible by 400. So 2024 and 2000 are leap years, while 1900 and 2100 are not. A person born on January 1, 2000 has lived 26 × 365 + 7 leap days = 9,497 days by January 1, 2026.
Weeks and weekdays
Divide the total days by 7 to get weeks and leftover days. 358 days is 51 weeks and 1 day. To count weekdays, you need to remove Saturdays and Sundays, and subtract any public holidays that matter to you.
Common uses
- School and sports age cutoffs
- Retirement and Social Security planning
- Insurance and medical forms
- Notice periods and contracts
- Countdowns to events, trips and deadlines
Common mistakes
- Forgetting the leap day
- Counting both the start and end date by accident
- Using 30 days for every month
- Using the wrong date format, since the US writes month first
Use the calculators
Enter a birth date in our Age Calculator for exact age and total days lived. Use the Days Between Dates Calculator for any two dates. Both run in your browser, and nothing you type is stored.
Example: days until a date
To count the days from October 8, 2026 to December 25, 2026: 23 days remain in October after the 8th, plus 30 days in November, plus 25 days in December, a total of 78 days.
Age rules differ by situation
Legal age is usually counted in full years from the birth date. Some situations, such as school cutoffs or insurance, use a specific cutoff date instead. In some cultures, traditional age counting is different from the age shown on official documents. Always follow the rule for the form or program you are dealing with.
Frequently asked questions
How do I find my age in months?
Multiply full years by 12 and add the extra months.
What if I was born on February 29?
In common practice, birthdays in non-leap years are celebrated on February 28 or March 1, depending on local rules.
Does the calculator include today?
The days are counted up to today, not including today.