Counting the number of days between two dates seems like it should be simple arithmetic, and mathematically it is, but manually counting through a calendar by hand is slow and error-prone, especially across months of different lengths or a leap year. The reliable method converts both dates to a single, continuous numeric scale first, then subtracts.
The continuous day-count method
Rather than counting month by month, accurate date-difference calculations convert each date into a single number representing its position on a continuous day count, essentially, how many days have elapsed since some fixed distant reference point (systems like the Julian day number or Unix epoch day count both work this way). Once both dates are expressed as numbers on this same continuous scale, finding the difference is pure subtraction, no need to separately track how many days are in each month you're crossing or whether any leap years fall in between, that complexity is already absorbed into how each date's number was calculated in the first place.
The inclusive vs. exclusive counting question
One genuine ambiguity in "days between two dates" is whether the start date, end date, or both should be counted. If a task runs from Monday to Friday, is that a 4-day span (exclusive counting, just the gap between the two dates) or a 5-day span (inclusive counting, counting both Monday and Friday as full days)? Both are mathematically valid interpretations of "days between," and different practical contexts (travel-day counting, project-duration counting, age calculations) default to different conventions, which is why it's worth checking which one a given calculator uses rather than assuming. The Date Difference Calculator makes this distinction explicit rather than leaving it ambiguous.
Related but distinct: working days, and adding time back onto a date
If what you actually need is the number of business days rather than raw calendar days, that's a related but separate calculation covered in our guide on working days in a date range, since it additionally needs to exclude weekends and possibly holidays. And if you're working the problem in the opposite direction, starting from one date and a known duration, and need to find the resulting date, that's the Add/Subtract Dates Calculator's job rather than a difference calculation.