Find days between dates

how many days in between two dates

Finding how many days in between two dates is a lookup, not a theory. You can find the span on a wall calendar by walking your finger from one square to the other, counting the boundaries you cross, not the squares you start on. You can find it in a date tool by entering both dates. You can find it by converting each date to an ordered day number and subtracting. All three hunt the same object: the gap.

What you must not do is find a month name in the middle and guess. “March to June” is not a day count until both dates have a day number and a year. March 31 to June 1 is a different gap from March 1 to June 30. The months are the scenery. The dates are the points.

If the two dates sit in different years, still find the gap by subtraction. Do not add 365 blindly. A leap day between them adds one. Absence of a leap day does not. Finding the days is checking the actual calendar that those years used.

Write both dates with years, in a form that cannot swap month and day. Confirm which date is earlier. Count midnights from the earlier date to the later date, or subtract day numbers. Add one only if you meant to find how many dates are in the inclusive list. If you meant weekdays, find the calendar gap first, then drop Saturday and Sunday.

Finding the span on a paper calendar

On a month grid, the cleanest find is to note the weekday of the start date. Full weeks from there are seven-day jumps. A remainder of two days is two more squares, not a new month rule. When the finger crosses into the next month, keep the same running total. The new month heading is not a reset.

Inclusive finding looks like circling every square from start through end and counting the circles. Exclusive finding looks like starting the count on the square after the start. If you circle both and also start counting on the start square while thinking of “days later,” you will find one extra day and then distrust the calendar. The calendar was fine. The circling mixed two jobs.

Finding the days between dates is hunting midnights. Months are just the drawers you open while you hunt.

Same day, next day, and the date that runs backward

Find a span of zero when both dates are the same and you want days later. Find a span of one when the dates are consecutive. Find a negative span when the “end” is earlier than the “start,” or swap them and report a distance. Reversed dates are not an error in the calendar. They are a direction.

If the two dates are civil dates in different time zones, check whether you meant those local calendar dates or the same instant. A place east of you can already be on the next date while your calendar still shows yesterday. Finding “the” date then needs a place name. A day count between two local dates does not need a clock; a day count between two instants does.