📅 Date Duration
Days between dates · business days · date math

Days Between Dates Calculator

Calculate duration, add or subtract days, or look up ISO week numbers — instantly.

Count mode:
Both start and end counted

Reverse: Which dates are in week N of year Y?

How Date Calculations Work

The calculator uses precise calendar arithmetic with JavaScript's Date objects, handling leap years, month-length variations, and timezone-safe date comparisons.

Calendar Days vs. Business Days

Calendar days count every day between two dates including weekends. Business days count only Monday through Friday, excluding any holidays you specify. For example, from Monday Jan 6 to Friday Jan 10 is 5 calendar days but also 5 business days (no weekends in between).

Inclusive vs. Exclusive Counting

Inclusive counting includes both the start and end dates (commonly used for project durations). Exclusive counting includes neither endpoint (used for measuring the gap between events). A range of Jan 1 to Jan 3 is 3 days inclusive, 1 day exclusive, or 2 days when counting just one endpoint.

Month-End Date Arithmetic

Adding months requires care at month boundaries. January 31 + 1 month would be February 31, which doesn't exist. The calculator clamps to the last valid day: February 28 (or 29 in leap years). This matches standard business and legal date conventions.

ISO Week Numbers

ISO 8601 defines week 1 as the week containing the year's first Thursday. Weeks run Monday to Sunday. Some years have 53 weeks. This system is used internationally for business planning and scheduling.

Frequently Asked Questions

A business day is any Monday through Friday that is not a public holiday. Weekends are never business days. The calculator lets you add a custom holiday list — no country-specific holidays are hardcoded.
The calculator subtracts start from end to get total milliseconds, divides by 86,400,000 to get days. Inclusive adds 1 (both endpoints counted); exclusive subtracts 1 (neither endpoint). Business days iterate day-by-day skipping weekends and your listed holidays.
Adding months sets the month field directly, then checks if the day overflowed (e.g., adding 1 month to Jan 31 produces Feb 31 which JavaScript auto-corrects to Mar 3). The calculator detects this overflow and clamps back to the last day of the intended month (Feb 28 or 29).
ISO 8601 week numbering defines week 1 as containing the year's first Thursday. Weeks run Monday to Sunday. A year has 52 or 53 ISO weeks. The standard is widely used in business planning across Europe and internationally.
Yes. In the Duration tab, enable Business Days Mode and use the holiday input to add up to 20 specific dates. Your list is saved in your browser between sessions.
When the result of adding months lands on a non-existent date (e.g., Jan 31 + 1 month = Feb 31), the date is clamped to the last valid day of that month — Feb 28 or Feb 29 in a leap year. This follows standard legal and business date conventions.