Work out recurring event dates instantly with clear inputs, formula shown and shareable results.
A fixed-interval series has n-1 gaps between n occurrences, which is where off-by-one errors creep in. Ten fortnightly events from 12 January 2026 span 126 days and finish on 18 May, not 140 days later. A 14 day cycle repeats about 26.1 times a year, which is why fortnightly payrolls have an occasional three-payment month.
Last occurrence
last = first + interval x (occurrences - 1)
Annual frequency
per year = 365.25 / interval
Because the first occurrence is at day zero. Ten events at 14 day spacing have nine gaps, so 126 days rather than 140.
Because 26 fortnights are 364 days, so the cycle drifts forward one or two days a year and eventually a 27th falls inside the calendar year.