How to Count Days Between Dates (and Why Answers Differ by One)
From Monday to Friday is 4 days or 5, depending on whether you count the end date; January 31 plus a month is February 29 in 2024; and a calculator that divides clock hours can lose a day at a daylight-saving change.
Counting Days: In or Out?
The most common date question has two right answers. From Monday to Friday is 4 days if you count the gaps between days (the usual way, and what subtracting dates gives you), but 5 days if you count every day you are there. Hotel nights and “days until” use the first; event days, rentals charged per day and “days of work” often use the second. Our Date Calculator counts the first way by default and has a checkbox to include the end date.
Months Are Not All the Same Length
Adding a month to January 31 has no perfect answer, because February has no 31st. The common rule, used by Python’s dateutil and most calendar software, moves the date to the last day of the shorter month: January 31, 2024 plus one month is February 29, 2024 (a leap year), and in 2023 it is February 28. The same rule means the time from January 31 to March 1 is “1 month and 1 day” even though it is only 30 days in 2024. When a deadline is written in months, check whether the contract or rule says how month ends are handled.
Why Some Calculators Are Off by a Day
A date calculator that subtracts clock times and divides by 24 hours can be wrong around daylight-saving changes, when a day is 23 or 25 hours long, and in time zones far from UTC. Counting whole calendar days avoids it: in our tests, with the browser set to New York time, March 10 to 11, 2024 (a 23-hour day) and November 3 to 4 (a 25-hour day) both counted as exactly one day.
Weekdays and Business Days
Counting weekdays (Monday to Friday) is a calendar question; counting business days also needs a list of holidays, which differs by country and employer. The date calculator gives the weekday count; for working days with public holidays removed, the Business Day Adder covers several countries’ holiday rules.
How We Checked
We compared the calculator with Python’s datetime, dateutil and NumPy on 5,000 random dates between 1900 and 2100: total days, years/months/days, weekdays, day names and additions all matched in every case, after fixing a bug the first run found in weekday counts before 1970.
Count days or find a date
Days between dates, weeks, months, weekdays; add or subtract days, weeks, months, years.
Frequently Asked Questions
How many days are there between two dates?
Do you count the start date or the end date?
What is one month after January 31?
Why is my date calculation off by one day?
Published Oct 14, 2026 by Derek Giordano