Do you count both the start and end date?
Subtracting two dates gives the gap between them and leaves the end date out. Counting the days themselves includes both ends and comes out one higher. Neither is wrong. Which one you need is set by your document, not by the tool, and most tools do not say which they picked.
Why two calculators disagree by one
Take 1 March to 5 March. How many days are between them? Four, because you cross four midnights. How many days is that, counting both ends? Five, because both ends are days.
The first is a difference. The second is a span. Subtraction gives you the difference; counting on your fingers gives you the span. Every off-by-one argument about dates is these two questions wearing the same words.
This site's days-between tool computes the difference by default and ships an Include end date checkbox that switches it to the span. You don't have to do the arithmetic yourself. You do have to know which one you want.
Which one does your situation need?
The right answer is written down somewhere, usually by someone else.
- Deadlines. A period of "within 30 days" commonly runs from the day after the triggering event. That's the rule in US federal civil procedure and in English civil practice, and it isn't universal: plenty of contracts and civil-law statutes count from the day of the act itself, and "clear days" provisions count differently again.
- Hotels, car rentals, hospital stays. These count nights, which is the difference. Check in on the 3rd, out on the 7th, four nights.
- Payroll and leave. These count days worked, which is a span. Monday to Friday is five days of leave, not four.
- Rental and subscription terms. Usually a span with the last day included, which is why a 30-day term starting on the 1st ends on the 30th.
When your document defines it, use its definition. When it doesn't, write down which one you used. Most disputes about a date aren't about arithmetic.
What about weekends and holidays?
Neither convention handles them. Both count every calendar day unless you switch to business days, which removes weekends and whatever holidays you tell it to remove. And in many systems a deadline landing on a weekend or public holiday rolls forward to the next working day, which is a third rule sitting on top of the first two. That one is in your document, not in any calculator.
Common questions
Is 1 March to 5 March four days or five?
Four as a difference, five as a span. If you're describing a stay, four. If you're describing how many days a task runs, five.
How do I tell what a tool is doing?
Enter two consecutive days. An answer of 1 is a difference. An answer of 2 is a span.
Does "one month later" work the same way?
No. A month isn't a fixed number of days, so "one month later" is a rule rather than a sum. 31 January plus one month is 28 February, or 29 in a leap year, because the 31st doesn't exist in February and the convention is to clamp to the last day. Our date calculator calls that constrain overflow and says so on the page.
Does a leap day count?
Yes, and any tool that subtracts calendar dates handles it. Anything that multiplies weeks by seven doesn't.
Related
The days between dates tool reports the difference and carries the Include-end-date toggle. The date calculator adds and subtracts from a date. And what "5 to 7 business days" means covers the version of this problem you meet in shipping windows.