Counting the days between two dates sounds trivial until you try it. Months have 28, 29, 30 or 31 days, leap years add an extra day every so often, and people disagree about whether the first and last days should count. A notice period, a visa stay, a pregnancy due date or a project deadline can all hinge on getting it exactly right.
This guide shows two reliable ways to count days by hand, explains the leap-year rule, clears up the inclusive versus exclusive question and covers business days. When you would rather skip the arithmetic, our date difference calculator gives the answer in days, weeks and months instantly.
How do you count the days between two dates by hand?
The month-by-month method is the easiest to do on paper. Count the days left in the starting month, add the full months in between, then add the days in the final month. From 15 March to 10 June 2026: there are 16 days from 15 to 31 March, then 30 in April, 31 in May and 10 in June. 16 + 30 + 31 + 10 = 87 days.
The day-of-year method is handy when both dates fall in the same year. Look up each date's position in the year (its ordinal day) and subtract. In 2026, 15 March is day 74 and 10 June is day 161, and 161 − 74 = 87 days, matching the first method.
- 31 days: January, March, May, July, August, October, December.
- 30 days: April, June, September, November.
- February: 28 days, or 29 in a leap year.
How do leap years affect the count?
A leap year adds 29 February, so any range that crosses the end of February in a leap year is one day longer. From 1 January to 1 March is 59 days in 2026 but 60 days in 2024.
The rule has three parts. A year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. That is why 2000 was a leap year but 2100 will not be. A normal year has 365 days and a leap year 366, and the calendar averages about 365.2425 days per year over a 400-year cycle.
Should you include the start and end date?
By default, most calculators and spreadsheets count the difference between dates, which excludes one end. From Monday to Friday of the same week is 4 days by this measure, the same way that from 1 o'clock to 5 o'clock is 4 hours.
Inclusive counting adds one day, so that both the first and last day are counted. Monday to Friday becomes 5 days. Use inclusive counting when every calendar day matters in its own right, such as a hotel stay measured in days on site, an event running from one date to another, or a leave request. The 87-day example above becomes 88 days inclusive. Contracts and legal deadlines often specify which method applies, so check the wording.
How to count business days between dates
Business days usually exclude Saturdays and Sundays. A quick way to count them is to take the number of full weeks and multiply by 5, then add the weekdays in any remaining partial week. From Monday 12 October to Friday 23 October 2026, inclusive, there are exactly two working weeks, or 10 business days.
Public holidays vary by country, region and even employer, so subtract them separately. Spreadsheets offer functions such as NETWORKDAYS that take a list of holidays, and our date difference calculator includes a business-days option for weekends.
How many days until a future date?
A countdown is just a date difference where the start date is today. From 12 October to 25 December 2026 is 74 days, which is 10 weeks and 4 days. Christmas Day falls on a Friday that year, which is useful to know when planning time off.
Because the starting point changes every day, a countdown is easiest to keep with a tool that reads the current date. Our days until calculator shows the days, weeks and hours remaining for any birthday, holiday or deadline, with a live countdown.
How to calculate an exact age in years, months and days
Ages are counted in calendar units rather than raw days. Work out the full years first, then the full months since the last birthday, then the remaining days. Someone born on 20 May 1990 is 36 years old on 20 May 2026, 36 years and 4 months on 20 September, and 36 years, 4 months and 22 days on 12 October 2026. On that day they have lived 13,294 days in total.
Birthdays on 29 February and month-end dates add extra edge cases, which is why an age calculator is the safer option for official forms.
If you want to know what share of the year has passed, divide the day number by 365 or 366 and use the steps in our guide on how to calculate percentages.
Tips for accurate date calculations
A few habits prevent most date errors, whether you count by hand, in a spreadsheet or with an online tool:
- Write dates in an unambiguous format such as 2026-10-12; 03/04 means 3 April in some countries and 4 March in others.
- Decide up front whether the end date should be included, and note it next to your answer.
- Count calendar dates rather than hours to avoid surprises from daylight saving time changes.
- Double-check any range that crosses February or a year boundary.
- In a spreadsheet, subtracting one date cell from another gives the days between them.
Key takeaways
- Count the days left in the first month, add the full months in between, then add the days in the last month.
- Leap years add 29 February: every year divisible by 4, except century years not divisible by 400.
- A standard date difference excludes one end; add one day for inclusive counting.
- Business days exclude weekends, and public holidays must be removed separately.
- For exact ages, countdowns and long ranges, use a calculator and keep dates in YYYY-MM-DD format.