Two people can calculate the same date range differently because one measures elapsed time while the other counts every calendar date in the range. State the endpoint convention before using a result for a deadline, subscription, project, or contract.
Elapsed days
Elapsed days measure the distance from the start date to the end date. August 1 to August 31 is 30 elapsed days because the clock has advanced 30 date changes.
Inclusive calendar days
An inclusive count includes both endpoints. August 1 through August 31 contains 31 calendar dates, so add one to the elapsed result when both dates qualify.
Business days are different
Weekends and holidays are not removed from a calendar-day result. Use the working-days calculator and the relevant holiday calendar when a schedule depends on business availability.
Check edge cases
Test a same-day range, a month boundary, a leap day, a weekend crossing, and a reversed range. These examples reveal whether the rule is inclusive, exclusive, or invalid for the situation.
Sources
The figures and rules in this guide are checked against:
- Introduction to Calendars — U.S. Naval Observatory, Astronomical Applications Department
- Leap Years — U.S. Naval Observatory, Astronomical Applications Department
- Rule 6. Computing and Extending Time (Federal Rules of Civil Procedure) — Cornell Law School, Legal Information Institute
- ISO 8601: Date and time format — International Organization for Standardization