How To Calculate Days Between Dates

Calendar arithmetic is awkward because months and years have irregular lengths. Counting days by hand means remembering which months have thirty-one days, which year is a leap year, and how the century rules work.

The reliable approach converts each date to a day number first. A day number is an integer that counts days from a fixed epoch, so the difference between two day numbers is exactly the number of days between the dates, with no special cases.

The standard day count is the Julian day number, used in astronomy since the sixteenth century. It is defined by a short formula that uses only integer division, so it can be evaluated exactly on a computer or by hand.

Converting a day number back to a calendar date is the inverse problem, and it has its own standard algorithm. It recovers the day, month and year in a few steps, again using nothing but integer division and subtraction.

The weekday comes almost for free. Because Julian day numbers run in a continuous sequence, taking the day number modulo seven gives the day of the week, with a fixed offset that puts Sunday at zero.

Adding days to a date is then trivial: convert the date to a day number, add the offset, and convert back. This handles month ends, year ends and leap days without any special-case logic.

Subtracting days works the same way with a negative offset. Going back from 1 March in a common year to 28 February is one day; going back from 1 March in a leap year is two, and the day-number method gets both right automatically.

The number of whole weeks between two dates is the day count divided by seven and rounded down. A span of ten days is one whole week plus three days.

The number of months between two dates is ambiguous, because months have different lengths. Dividing by the average month length gives a summary figure, but the exact answer needs the borrowing method used for ages.

Business-day counts are a separate problem. Weekends and public holidays have to be excluded, which the simple day difference does not do.

Interest and term calculations often use a day-count convention rather than the plain difference. The actual over three-sixty-five convention divides the actual day count by three hundred and sixty-five, while other conventions assume thirty-day months.

Because the calculation is exact, it is useful for contract terms, notice periods, warranty windows, project schedules and anything else that turns on a date.

The calculator takes both dates as day, month and year numbers, so enter each part in its own box. The offset is the number of days to add to the from date, and it can be negative.