Two ways to count
The duration between two dates can be read two ways, and both are useful: as a calendar breakdown (2 years, 3 months, 5 days) and as a total count (825 days). The calendar breakdown respects that months and years have different lengths; the total is what you want for interest, age in days, or "how many sleeps".
How the calendar breakdown works
Counting whole years, then whole months, then leftover days — borrowing from the previous month when the day-of-month goes negative, exactly the way you'd do it by hand. Because month lengths vary, "1 month" isn't a fixed number of days; the total-days figure is the unambiguous one when precision matters.
Business days
The weekday count tallies Monday through Friday across the span — handy for rough project timelines. It does not know about public holidays, which vary by country and year, so subtract those separately for exact working-day counts.
Frequently asked questions
- Are both endpoints included?
- The total-days figure is the count between the dates (end minus start), so a Monday-to-the-next-Monday span is 7 days. Add one if you mean to count both endpoints inclusively.
- Does it handle leap years?
- Yes — the arithmetic uses real calendar dates, so February 29 and leap years are counted correctly.
- What about time zones and daylight saving?
- Deliberately ignored. Dates are treated as plain calendar days in UTC, so a "1 day later" never lands on the wrong day because the clocks changed. For timestamp math down to the second, use the Unix timestamp converter.
- What's the ISO week number?
- The ISO 8601 week of the year (weeks start Monday; week 1 contains the first Thursday). It's the week numbering most calendars and businesses in Europe use.