How to calculate age and the days between two dates
Working out an exact age, or how many days are left until a deadline, sounds easy until months have different lengths and leap years get involved. Here is how the counting works, and how to avoid the usual mistakes.
How age is counted
A person’s age is the number of complete years since their birth date. You turn a year older on the birthday, not before it.
For a more exact answer, add the extra complete months and the leftover days. Someone born on 15 August 1990 is, on 21 October 2026, 36 years, 2 months and 6 days old.
Working it out by hand
- Subtract the birth year from the current year: 2026 − 1990 = 36.
- If the birthday has not happened yet this year, take one year off. Here it already has, so 36 stays.
- Count the complete months from the last birthday (15 August) to today: 15 August to 15 October is 2 months.
- Count the days left over: from 15 October to 21 October is 6 days.
Why months make it tricky
Months have 28 to 31 days, so there is no single answer to the question “how many days is a month?”. Calculators handle this by counting complete calendar months and then the real days that remain.
This is also why the number of days between dates is not simply the difference in months multiplied by 30. From 1 January to 1 March 2026 there are 59 days (31 in January plus 28 in February), not 60.
Finding the days between two dates
The cleanest method is to give each date a running day number and subtract. Calculators do this using the real calendar, including leap years, so you do not have to add up each month yourself.
Decide whether the end date counts. From Monday to Friday is 4 days apart, but 5 calendar days if you include both Monday and Friday. A project that runs “from the 1st to the 10th inclusive” is 10 days long, not 9.
Leap years and 29 February
A leap year adds a 29th day to February. A year is a leap year if it divides by 4, except that years divisible by 100 are not leap years unless they also divide by 400. So 2000 and 2024 were leap years, while 1900 and 2100 are not.
People born on 29 February only have a calendar birthday every fourth year. In other years, some places treat 28 February as the birthday and others treat 1 March. Rules differ between countries, so check local law if it matters, for example for a legal age limit.
Working days instead of calendar days
For deadlines, delivery times and notice periods you often need working days, which skip weekends and sometimes public holidays. Five working days from a Wednesday lands on the following Wednesday, because the weekend is skipped.
The working days calculator on Toolstia counts working days between two dates and lets you choose which days are the weekend and add holidays to skip.
Day of the week for a date
If you only need to know which weekday a date falls on, the day of the week calculator gives it for any date from year 1 onwards, together with the day of the year and the ISO week number.
Tips to avoid mistakes
- Write dates in an unambiguous order. 03/04/2026 is 3 April in much of the world and 4 March in the United States. Using 2026-04-03 avoids the confusion.
- Decide beforehand whether the start or end date is included.
- Use calendar dates only, not times, so time zones do not shift the answer by a day.
- Remember that a month is not 30 days when planning around a specific date.