What is the Date Difference Calculator?
This calculator counts how many days fall between two calendar dates. It can count every calendar day or only business days (Monday to Friday, with an option to include Saturday). It works for both future and past dates, making it handy for contract deadlines, shipping windows, project timelines, payment terms, age, and countdowns. The math uses the proleptic Gregorian calendar, so leap years are handled exactly.
How to use it
Pick a Date Format that matches how you type your dates: US (mm/dd/yyyy), EU (dd/mm/yyyy), or ISO (yyyy-mm-dd). For US and EU you may separate parts with a slash, dot, or dash (for example 6/15/2026, 6.15.2026, or 6-15-2026); ISO uses dashes only. Enter the Start Date and End Date. Tick "Count Business Days Only" to exclude weekends, and tick "Saturday is a Business Day" if Saturdays should also count. The order of the two dates does not matter — the result is always a positive gap.
The formula explained
Each date is converted to a Julian Day Number (JDN), a unique integer serial value. Subtracting the two serials gives an exact, calendar-correct day count regardless of month length or leap years.
$$\text{JDN} = d + \left\lfloor\frac{153m+2}{5}\right\rfloor + 365y + \left\lfloor\frac{y}{4}\right\rfloor - \left\lfloor\frac{y}{100}\right\rfloor + \left\lfloor\frac{y}{400}\right\rfloor - 32045$$$$\text{Days} = \left| \text{JDN}(\text{end}) - \text{JDN}(\text{start}) \right|$$For all-days mode the answer is the absolute difference of the two serials. For business-days mode the calculator walks each serial value after the earlier date through the later date, determines the weekday, and counts only the days you selected.
Worked example
From 6/15/2026 to 7/15/2026: June 15 to June 30 is 15 days, plus July 1 to July 15 is 15 days, for a total of 30 days.
$$\text{Days} = 15 + 15 = 30$$In business-days mode (weekends excluded) the same span yields 22 business days; if Saturday counts too, the four Saturdays in the span are added back for 26.
$$22 + 4 = 26$$
FAQ
Does the count include the start day? No. The result is the gap between dates (an exclusive count), so 2/1 to 2/5 returns 4 days, not 5.
Does the date format change the answer? No. The format only controls how your typed text is parsed. 15/06/2026 (EU) and 06/15/2026 (US) are the same date and give identical results.
Are leap years handled? Yes. Feb 29, 2024 is valid; the serialization automatically accounts for the extra day, while an impossible date like Feb 29, 2023 is rejected.
