Date Calculator: Days Between Dates & Date Addition Tool

Formula Explained

Variables Explained

Tips & Insights

• 1Use the Days Between mode to calculate exactly how many days until your next vacation, birthday, or deadline.
• 2Use negative values in the Add Days field to subtract days from a date (e.g., -90 gives you the date 90 days before).
• 3For planning purposes, add 2-3 buffer days to account for weekends and holidays when calculating project deadlines.
• 4The total days count includes both the start and end date, so a 7-day trip from Monday to Sunday counts as 7 days.

Key Terms & Definitions

Calendar Days

All days of the week including weekends and holidays. Used for date calculations in planning and scheduling.

Leap Year

A year with 366 days (February 29th), occurring every 4 years. Years divisible by 100 are not leap years unless also divisible by 400.

Data Sources & References

Our calculations are based on established financial and scientific formulas. The following sources inform the default values and methodology used in this calculator:

• National Institute of Standards and Technology (NIST) — Calendar Dates and Time

Date Difference and Business Days Count

To count the days between two dates, work out the full calendar days and, if you need working days, subtract weekends and optionally holidays. Project planning usually uses business days only. For example, 30 calendar days starting on a Monday typically equals about 21-22 business days depending on holidays. Always decide whether to include the start date in your total, and confirm the convention when someone says business days, because a 10-business-day order may include or exclude the current day. Note that daylight-saving shifts can move the wall-clock difference by an hour but not the calendar-day count, and that dates written day/month can be misread as month/day. Adding months is not symmetric arithmetic because months differ in length, so January 31 plus one month has no straightforward calendar answer. As an example, from September 5 to December 3 is 89 calendar days and about 62 business days.

Adding and Subtracting Days From a Date: Worked Examples

Date arithmetic trips people up in two places: calendar days versus business days, and month-end edge cases. Adding calendar days is pure counting — every day counts, weekends included. Adding business days skips weekends (and, in most conventions, holidays you specify). The two diverge fast: 20 calendar days is 20 days, but 20 business days is 28 calendar days — nearly a week and a half longer.

The month-end case matters for billing and contracts: January 31 plus one month cannot be February 31, so the convention is to clamp to the last day of the month (February 28, or 29 in leap years) — which is what courts and most accounting standards assume. When a deadline says "within 30 days," it almost always means calendar days; "within 30 business days" roughly doubles the real time once weekends and a holiday or two land inside the window. The calculator above handles both modes, including the month-end clamping rule.

Related Calculators