Formula
Final date = Start date + (Occurrences − 1) × fixed interval in daysDirect answer
Start with occurrence one, then add the fixed calendar-day interval once for every remaining occurrence. ISO 8601 covers unambiguous Gregorian-date, duration and recurring-interval representations; this calculator applies a deliberately narrower fixed-day schedule.
Variables and formula
- Start date: occurrence one in the schedule.
- Interval between dates: whole calendar days between consecutive occurrences.
- Occurrences: total dates to include, including the start date.
Final date = start date + (occurrences − 1) × interval days
Reproducible example
Starting on 2026-08-24 with an interval of 14 days and three occurrences gives 2026-09-21. The final date is 28 calendar days after occurrence one.
Interpretation
The final date is the last item in one evenly spaced sequence. It is not a monthly rule: an interval of 30 days does not mean the same day in every calendar month.
Assumptions and limitations
- Every gap has the same whole number of calendar days.
- The start date counts as the first occurrence.
- Time zones, holidays, business-day adjustments and non-Gregorian calendars are excluded.
Common mistakes
- Adding one interval too many by treating the start as occurrence zero.
- Treating a fixed number of days as a monthly recurrence.
- Using this result when an agreement specifies working days or local holiday treatment.
Assumptions
- Each recurrence is separated by the same whole number of calendar days.
- The start date is occurrence one.
- Time zones, clock times, monthly rules, holiday adjustments and calendar systems other than Gregorian are excluded.
Sources and methodology
CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.
- ISO 8601 Date and Time Format; International Organization for Standardization; accessed Aug 24, 2026