How many days until a deadline, how long a contract ran, how old someone is in days: counting days sounds trivial until month lengths, leap years and the question "do I count the last day?" get involved. Here's how to count the days between two dates by hand, in Excel or Google Sheets, or instantly with a calculator, plus how to add or subtract days from a date.
Example used throughout: from 27 September 2026 to 25 December 2026 there are 89 days.
Do you count the last day?
This is the question behind most "the calculator is wrong" moments. There are two conventions, and both are correct for different jobs:
- Days between (end date not counted): the end date minus the start date. A hotel stay from Monday to Friday is 4 nights. Deadlines, notice periods and "days until" all work this way.
- Days inclusive (both dates counted): add 1. A training course that runs from Monday 5 October to Friday 9 October 2026 lasts 5 days. Event durations, leave requests and "how many days did it run" usually work this way.
Counting days by hand
Split the period into three parts: the days left in the start month, the full months in between, and the days in the end month.
- Days left in September after the 27th: 28, 29 and 30, so 3.
- Full months in between: October (31) + November (30) = 61.
- Days in December up to the 25th: 25.
- Total: 3 + 61 + 25 = 89 days.
For this you need the length of each month. The old rhyme still works: "Thirty days hath September, April, June and November; all the rest have thirty-one, excepting February alone", which has 28 days, or 29 in a leap year.
Leap years
A leap year adds 29 February, making the year 366 days long. The rule has three parts:
- a year divisible by 4 is a leap year (2028, 2032);
- except years divisible by 100, which are not (1900, 2100);
- except years divisible by 400, which are (2000).
The next leap year is 2028. Any period that crosses 29 February of a leap year gets one extra day.
Years, months and days
Sometimes a count of days isn't what you want. Ages and contract lengths are usually given in years, months and days. Someone born on 15 March 1990 is, on 27 September 2026, 36 years, 6 months and 12 days old, or 13,345 days.
Months are the tricky part because they have different lengths. The fair way to count is to step forward one whole month at a time from the start date, then count the leftover days. When a month is too short for the start day, the step lands on its last day. For example, one month after 31 January is 28 February (29 in a leap year), so 31 January to 1 March is 1 month and 1 day. This is the same approach spreadsheet functions such as EDATE use.
Adding or subtracting days
To find a date in the future, add the days to the start date. 90 days after 27 September 2026 is Saturday, 26 December 2026. Count forward month by month: 3 days to the end of September, 31 in October (34 so far), 30 in November (64), then 26 more into December.
Adding months follows the same end-of-month rule as above: 31 August minus 6 months is 28 February, not 3 March.
In Excel and Google Sheets
Dates in spreadsheets are stored as numbers, so subtraction just works. With the start date in A2 and the end date in B2:
| What you want | Formula |
|---|---|
| Days between | =B2-A2 |
| Days inclusive | =B2-A2+1 |
| Whole years between | =DATEDIF(A2,B2,"Y") |
| Remaining whole months after the years | =DATEDIF(A2,B2,"YM") |
| Working days (Mon–Fri, both dates included) | =NETWORKDAYS(A2,B2) |
| Date 90 days later | =A2+90 |
| Date 10 working days later | =WORKDAY(A2,10) |
| Same day one month later | =EDATE(A2,1) |
Format the result cell as a number (for day counts) or as a date (for the "later" formulas). Microsoft notes that DATEDIF with the "MD" unit can return inaccurate results, so for the leftover days it is safer to subtract dates directly.
A catch when counting in hours: daylight saving
If you calculate with date-times rather than dates, a daylight-saving change can make one day 23 or 25 hours long. Dividing a time difference by 24 hours then comes out a fraction short, and rounding down loses a whole day. For example, from 1 March to 1 April 2026 is 31 days, but in the UK the clocks go forward on 29 March, so the gap is 30 days and 23 hours. Always count calendar dates, not elapsed hours, when you want a number of days.
Use the Date Calculator
Our free Date Calculator does both jobs. In Date Difference mode it shows the number of days, weeks, months and years between two dates, plus a years, months and days breakdown, with an option to include the end date. In Add / Subtract Time mode it adds or subtracts any combination of years, months, weeks and days, and tells you the weekday of the result. It counts calendar days, so daylight-saving changes never throw it off.
Frequently asked questions
How many days are there in a year?
365 in a common year and 366 in a leap year. On average, a year in the Gregorian calendar is 365.2425 days long.
How many weeks are there between two dates?
Divide the number of days by 7. For example, 89 days is 12 weeks and 5 days.
How do I count only working days?
Use =NETWORKDAYS(start, end) in Excel or Google Sheets. It counts Monday to Friday, including both dates, and can also skip a list of public holidays you provide.
Why do two calculators give me different answers?
Usually because one counts the end date and the other doesn't, or because they handle month-ends differently. Check whether you need "days between" or "days inclusive".