About the date difference calculation
Counting days between dates sounds trivial until leap years, month lengths and daylight saving get involved. This counts whole calendar days, ignoring time of day, so a date in late March and one in late October are not thrown off by an hour of clock change.
Formula
Days = (UTC midnight of end − UTC midnight of start) ÷ 86,400,000
Normalising both to UTC midnight is what makes daylight saving irrelevant.
Working days exclude Saturday and Sunday, counting from the start date and excluding the end date.Worked example
1 January 2026 to 25 December 2026 is 358 days — 51 weeks and 1 day, of which 256 are weekdays.
Frequently asked questions
Does the count include both the start and end dates?
No. It counts the days elapsed between them, so 1 January to 2 January is one day. If you need both endpoints included — counting a holiday, for instance — add one.
Are public holidays excluded from working days?
No. Only Saturdays and Sundays are excluded, because public holidays vary by country, region and year. Subtract your own local holidays from the working-day figure.
Why does the calendar breakdown not match days ÷ 30?
Because months are not 30 days. The years-months-days figure counts real calendar months of 28 to 31 days, which is why it cannot be derived from the day count alone.
Sources
- UTC-normalised Gregorian date arithmetic