1. Counting one end or both
From Monday to Friday is 4 days if you count the days after Monday, and 5 if you count both. We give 4 unless you tick “Count both dates”. More in inclusive vs. exclusive counting.
2. Adding a month to the 31st
January 31 + 1 month has no exact answer, because February has no 31st. Most tools, including ours, stop at the last day of the month: Sunday, February 28, 2027. Others carry the extra days over and answer March 3.
3. Days, weeks and months are different lengths
From Saturday, October 31, 2026:
- 90 days → Fri, Jan 29, 2027
- 13 weeks (91 days) → Sat, Jan 30, 2027
- 3 months → Sun, Jan 31, 2027
All three are “about three months”, and each lands on a different day.
4. Which days are business days
Thirty business days after Saturday, October 31, 2026 is Friday, December 11, 2026 when only weekends are skipped. Add the public holidays of any country and the date moves later, by a different amount in each country. Some calculators silently include one country's holidays; ours skips only the days you choose.
5. Which week is week 1
Under ISO 8601, used in much of Europe and Asia, weeks start on Monday and week 1 is the week with the year's first Thursday. In the US system, weeks start on Sunday and week 1 is the week with January 1. On January 1, 2027 the ISO week is 53 (of 2026), while the US week is 1.
6. What “today” means
At 10 p.m. on October 31 in New York it is already November 1 in London and Tokyo. A calculator that uses the server's date can be a day off for you. Ours uses the date on your device.
Which answer is right?
The one that follows the rule you need. Contracts, laws and payroll systems often define their own counting rules, so check them for anything important. Every rule we use is listed on how we calculate.