Counting days between dates is easy when both dates are in the same month and surprisingly error-prone when they are not. These methods work by hand and match what the calculators do.
Method 1: count days across months
Add the days remaining in the first month, the days in each full month between, and the days into the last month.
Example: October 10, 2026 to January 1, 2027
- Rest of October after the 10th: 31 - 10 = 21 days.
- November: 30 days.
- December: 31 days.
- Into January: 1 day.
Total: 21 + 30 + 31 + 1 = 83 days.
Method 2: day of the year
Number each date by its position in the year, then subtract. October 10 is day 283 in a common year (365 days). To handle two different years, add the days left in the first year to the day number in the second.
Leap years
A leap year has 366 days and a February 29. Leap years are divisible by 4, except century years not divisible by 400, so 2024 and 2028 are leap years, 2100 is not and 2000 was. From February 28 to March 1 is 2 days in 2024 and 1 day in 2025.
Include the end date or not?
"Days between" normally excludes one end: Monday to Friday is 4 days. If you are counting the days an event runs, including both ends gives 5. Decide which you need. The days between dates calculator has a checkbox to include the end date.
Weeks, months and years
- Weeks: divide the days by 7 and keep the remainder. 29 days is 4 weeks and 1 day.
- Months: step forward whole calendar months from the earlier date for as long as you do not pass the later date, then count leftover days.
- Years: 12 whole months make a year.
Month ends need care. January 31 plus one month is February 28 (or 29 in a leap year), because February has no 31st. January 31 to March 1, 2026 is 1 month and 1 day, which is 29 days in total.
Dates with clock times
When times matter, use the time duration calculator. On the weekends when daylight saving time starts and ends, a calendar day has 23 or 25 hours, so counting by hours can differ from counting by days.