Figure out days between dates

how many days in between two dates

How many days sit in between two dates depends on a choice you make before you count. The ordinary difference is the number of midnights you cross. Leave on the 1st and arrive on the 8th, and you have crossed seven midnights. That is the figure most calendars, date fields, and programming libraries return when you subtract one date from another.

The other common figure includes both calendar dates as members of the range. The 1st through the 8th then names eight dates. You get that number by taking the ordinary difference and adding one. Neither result is a trick. They answer different sentences: “how many days later” versus “how many dates does this stretch cover.”

Write the two dates in an order that cannot flip month and day. A numeric form with the year first, or a month name written out, both work. Then name which end you are counting. If you skip that sentence, two careful people can figure out two honest answers and still think the other person cannot add.

Put the earlier date first and the later date second. Subtract: later minus earlier. That is the ordinary span. If both dates belong in the range, add one. If the later date is actually earlier, the ordinary span is negative; the distance without direction is the absolute value.

Where the extra day hides

People lose a day when a phrase sounds inclusive and the arithmetic is not. “From Monday to Friday” in ordinary speech often means you are present on Friday. The subtraction Monday to Friday is four days later. Inclusive dates would be five. Hotel nights usually follow the subtraction: check in on the 1st and check out on the 3rd, and you have stayed two nights, even though three dates appear on the invoice as calendar squares.

Leave, school terms, and medicine courses often want the inclusive figure. Deadlines that say “within ten days after” often want the exclusive figure, starting the count on the following date. The words after, through, starting, nights, and business days are doing real work. Figure those words out before you figure the number out.

If two people disagree by exactly one day, they are arguing about the start date, not about subtraction. Same-date to same-date is zero days later and one date if you count the day itself. That sounds fussy until a form asks for the length of an event that begins and ends on one afternoon.

Leap days and months that will not sit still

You cannot turn a month into a fixed number of days. February has 28 days, or 29 in a leap year. April, June, September, and November have 30. The other months have 31. A span that crosses February 29 is one day longer than the same numbered span in a common year. The Gregorian leap rule is the one civil calendars use: a year divisible by 4 is a leap year, except a century year must also be divisible by 400. So 2000 is a leap year and 1900 is not.

Do not divide a long span by 30 and call the result months, then convert back, if you need a day count. Figure the days directly from the two dates. Weeks are cleaner than months: seven days make a week, so a remainder after dividing by seven is leftover days, not a mystery.