Calculating dates between days

how many days in between two dates

When you already have two dates, calculating days between dates is subtraction. The later date minus the earlier date is how many days in between two dates in the ordinary sense. Add one only if the question is how many calendar dates the range contains, not how many days later the second date falls.

The title of this page also runs the other way: you are given days, and you want dates. Start at a known date and move forward or backward by a whole number of days. Ten days after the 1st is the 11th in a month long enough to hold it. Ten days before the 1st is a date in the previous month. You do not convert to weeks first unless you want weeks in the answer. Weeks are a display. The unit you are moving in is the day.

Keep the time of day out of it unless the question is elapsed hours. A calendar date has no hour. Calculating a date from a day count will not tell you whether a meeting is in the morning. It will tell you which square of the calendar you land on.

Fix the starting date, including the year. Decide forward or backward. Add or subtract the number of days as a whole number. Let the month and year roll when the day number runs off the end of the month. If you needed the gap between two known dates instead, subtract those dates and stop.

Calculating days between dates in Excel

Calculating days between dates in Excel is the same subtraction as on paper, with serial numbers doing the rolling of months for you. End cell minus start cell, or DAYS, or DATEDIF with "d", returns the ordinary day gap. To go the other way — from days to a date — add the day count to a date cell: a date plus 40 is the date 40 days later. A date minus 40 is 40 days earlier. Format that result as a date.

Do not use a month-adding function when the instruction was “90 days.” Adding three months is a different calculation. It follows month lengths and end-of-month rules, not a day count. If a formula used EDATE, the result is a calendar-month shift, not a 90-day shift. Read the function name before you trust the cell.

WORKDAY and NETWORKDAYS belong to work-day arithmetic. They skip weekends and, with a list, holidays. Mixing them into a plain day calculation is how a “40 days later” date quietly becomes a later date than the calendar asked for.

Calculating a date from a pile of days is easy until February is in the path, and then the month does the rolling for you whether you remembered leap day or not.

When the month cannot help

There is no honest shortcut that says “a month is thirty days, so ninety days is three months later.” January 31 plus one month is not a day that exists in February. Software that adds months has to pick a rule: last day of February, or overflow into March. A day-count calculation never meets that fork. It only meets actual days.

If you are calculating both a day span and a month span for the same two dates, report them as two answers. They will not stay in a fixed ratio. That is not a rounding problem. That is the calendar.