The quick answer: decide what you are counting first
You have two dates and need the days between them, and the answer depends on one question: does the first day count, the last, or both? The days between dates calculator subtracts the earlier date from the later one, the way a spreadsheet does, and one switch counts both ends. Here is one pair of dates, Monday March 4 to Friday March 8, 2024, asked five ways:
| The question | Answer | Which tool |
|---|---|---|
| Days that pass (end minus start) | 4 | Days between dates |
| Calendar days covered, both ends counted | 5 | Days between dates, end date switch on |
| Working days, both ends counted, Saturday and Sunday off | 5 | Working days calculator |
| Working days, leaving out the start date | 4 | Working days calculator |
| The date 5 working days after Monday March 4 | Monday, March 11 | Working days calculator |
- Enter both dates, picked from the calendar or typed in the order the box shows.
- Leave Count the end date too off for the days that pass, or switch it on when both days count.
Do you count the first day?
It is the fence post problem. Five posts have four gaps between them, and Monday to Friday is five days but four days that pass. Neither answer is wrong. They answer different questions.
- Leave the end date out for how long until a date, how old something is, or the number of nights. Check in on Friday March 1 and out on Monday March 4 is 3 nights.
- Count both ends when the words say inclusive, or from the 1st to the 5th. March 1 to March 31 is 30 days that pass and 31 calendar days covered.
If a contract or form states the count in words, follow its wording and set the switch to match.
Weeks, months and leap years
A week is 7 days, so 30 days is 4 weeks and 2 days and 365 days is 52 weeks and 1 day. Months are not a fixed length: March 1 to April 1 is 31 days, and April 1 to May 1 is 30. January 31 to March 1 is 1 month and 1 day, and 29 days in 2023 but 30 in 2024.
A year is 365 days, or 366 across February 29. January 1, 2024 to January 1, 2025 is 366 days, while the same dates a year earlier give 365. The date difference calculator counts every leap day and also gives years, months and days.
Days and months give different dates, which matters when a rule is written in one and you count in the other:
| From March 1, 2024 | Result |
|---|---|
| Plus 30 days | March 31, 2024 |
| Plus 1 month | April 1, 2024 |
| Plus 90 days | May 30, 2024 |
| Plus 3 months | June 1, 2024 |
Working days: your days off, your holidays
A working day is a day that is neither a weekly day off nor a holiday you list. The working days calculator starts with Saturday and Sunday off, and you can pick any weekly days off, such as Friday and Saturday, or none. It knows no public holidays and never guesses: you enter them, one per line or as a range, because holidays differ by country, region and employer. A holiday on a weekly day off is not taken away twice.
March 1 to April 1, 2024 with both dates counted is 32 calendar days, and 10 are Saturdays or Sundays, so there are 22 working days. If Friday March 29 is also a day off, it is 21. A calendar year has between 260 and 262 weekdays before holidays: 2023 has 260 and 2024 has 262.
Counting between dates and adding days are different questions. Between two dates both ends count by default, like NETWORKDAYS in a spreadsheet. When you add working days the start date is never counted, like WORKDAY, so 5 working days after Monday March 4 is Monday March 11.
Calendar and working days drift apart fast. 30 calendar days from Friday March 1, 2024 ends on Sunday, March 31. 30 working days ends on Friday, April 12, exactly 6 weeks after the start. A carrier, bank or contract may mean something different by business day, and the calculator only follows the rules you set.
Adding or subtracting days, and the month end rule
To find a date a number of days away, use add days to a date. The day after the start date is day 1, so 1 day after a Monday is Tuesday. If a rule counts the start day itself as day 1, add one day fewer. 30 days after January 15 is February 14, and 365 days after January 1, 2024 is December 31, 2024, because 2024 has 366 days.
Months and years follow the calendar instead. The add and subtract calculator applies years and months first and weeks and days second, and when the new month is too short it uses that month's last day. January 31 plus 1 month is February 29 in 2024 and February 28 in other years.
That rule can stop a round trip: March 31, 2024 minus 1 month is February 29, and adding 1 month back gives March 29, not March 31. Add or subtract the whole amount in one go when exactness matters. In a spreadsheet, a date plus 90 is =A1+90 and the gap between two dates is =B1-A1.
Ages, countdowns and hours
The age calculator counts whole years, then months, then days, and a new year of age begins on the birthday itself. Someone born on March 14, 1992 is 34 years, 6 months and 22 days old on October 6, 2026, which is 12,624 days. A February 29 birthday is counted on February 28 in years that are not leap years, and some places use March 1, so check the rule if a legal age matters. Unlike the other date tools, it never writes a date of birth into the page address.
For a countdown, put today in the start box and the event in the end box; the answer changes tomorrow. To see how far through a period you are, divide the days passed by the total, as in percentage calculations explained: 45 days into March 1 to May 30, which is 90 days, is 45 ÷ 90 = 50%.
For hours rather than days, the time duration calculator finds the time between two clock times. 9:00 AM to 5:30 PM is 8 hours 30 minutes, or 8.5 as a decimal, and 7:30 is 7.5 hours, not 7.30. It is plain clock arithmetic and does not know about daylight saving or time zones.
What goes wrong
- Date order. 03/04/2024 is March 4 in month first order and April 3 in day first order. Type dates in the order the box shows and check the weekday: March 4, 2024 is a Monday, April 3 a Wednesday. Excel can also turn the dates in a CSV export into plain numbers, and the guide to converting CSV to Excel explains how to avoid that.
- An off by one. Decide whether the start or end day counts, then use the same rule for every date in the document.
- Calendar days against working days, or months against 30 days. A 30 day notice and a 30 working day notice end weeks apart, and three months from March 1 is June 1 while 90 days is May 30.