How to count the days between two dates (and when to add 1)
Subtract the earlier date from the later one. From October 5 to December 25, 2026 that gives 81 days. That's the plain subtraction answer, and it's what "how many days until Christmas?" means. Add 1 only when both the first and the last day should be counted, as in "October 5 through December 25": that's 82 days.
Pick the count that fits the question
The two answers differ by whether the start date is day 0 or day 1. Using June 1 to June 5 as the example:
| Question | How to count | June 1 to June 5 |
|---|---|---|
| How many days until June 5? | Subtract | 4 days |
| How many nights is the hotel stay? | Subtract | 4 nights |
| How long ago did it start? | Subtract | 4 days |
| How many days does the trip last? | Subtract, then add 1 | 5 days |
| How many days did I work, June 1 through June 5? | Subtract, then add 1 | 5 days |
It's the same trap as counting fence posts: a fence with 4 sections has 5 posts. Subtraction counts the gaps between dates, and the inclusive count counts the dates themselves.
Counting down to a date
A countdown is plain subtraction, so it reaches 0 on the day itself. On October 5, 2026 there are 81 days left until Friday, December 25. On December 24 there is 1 day left, and on December 25 the count is 0. If a countdown shows 82 on October 5, it is counting today as well. The countdown calculator uses subtraction.
81 days is also 11 weeks and 4 days, or 2 months and 20 days. The months are counted from date to date (October 5 to December 5 is two months), not by dividing by 30.
Why dividing by 30 goes wrong
Months have 28 to 31 days, so one division can't turn days into months. March 1 to April 15, 2026 is 45 days. Divided by 30 that looks like a month and a half, but on the calendar it is 1 month and 14 days: March 1 to April 1 is one month, then 14 more days. Month ends cause the biggest surprises, which is covered in what happens when you add a month to January 31.
Leap years add a day
February 1 to March 1 is 28 days in 2026 and 29 days in 2028. A whole year, January 1 to December 31, is 364 days by subtraction in 2026 (365 counting both ends) and 365 days in 2028 (366 counting both ends). A year is a leap year when it divides by 4, except century years that don't divide by 400: 2000 was a leap year and 2100 won't be.
In a spreadsheet
Excel stores dates as serial numbers, so =B2-A2 gives the subtraction count and =B2-A2+1 the inclusive one. The DAYS function does the same subtraction; in Microsoft's example, =DAYS("15-MAR-2021","1-FEB-2021") returns 42.
If you need working days rather than calendar days, see how to count business days.
Count the days between two dates →