Counting days
“How many days between the 1st and the 3rd” has two reasonable answers, 2 or 3, depending on whether both ends count. This calculator counts the elapsed time: from midnight on the 1st to midnight on the 3rd is 2 days. If you need an inclusive count (days of a hotel stay, days of a contract) add one. The same applies to business days: Friday to the next Friday is five working days.
Calendar arithmetic
Years and months have variable length, so “one month after 31 January” has no exact answer. The convention here, shared by most libraries, is to clamp to the end of the shorter month: 31 January + 1 month = 28 February (29 in a leap year). The difference mode works the other way: it finds the largest whole number of months that fits, then counts the remaining days, so 31 January to 1 March is 1 month and 1 day.
ISO weeks
ISO 8601 numbers weeks Monday to Sunday, and week 1 is the week containing the first Thursday of the year. That means 1 January can belong to week 52 or 53 of the previous year, and a year can have 53 weeks. Payroll, logistics and much of European business planning use ISO weeks; the US retail calendar does not. The results show the ISO week for both dates.
Business days
Business days here are Monday to Friday. Public holidays vary by country and year and are not subtracted; most countries have 9–14 a year, roughly one every month.
Questions
How many days are there between two dates, inclusive?
Take the calculator’s result and add 1. From 1 to 3 January is 2 days elapsed, 3 days inclusive.
What is 90 days from today?
Use the add mode with 90 in the days field; the start defaults to now. The result shows the weekday and ISO week as well.
How are leap years handled?
Correctly: 29 February exists in years divisible by 4, except centuries not divisible by 400. Differences across a leap day include it; adding a year to 29 February clamps to 28 February.
Why does adding a month to 31 March give 30 April?
April has 30 days, so the date is clamped to the last day of the month. Adding another month then gives 30 May, not 31 May; apply the full two months at once if you need the original day preserved.