Date and time
Build a calendar recurrence or inspect a rule, with an explicit start, timezone and date preview.
BYDAY: MO,WE or 2TU,-1FR (ordinals only monthly/yearly). BYMONTHDAY: −31…−1, 1…31; BYMONTH: 1…12; BYSETPOS: −366…−1, 1…366 and another BY filter. Lists use commas; blank filters are omitted.
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Start: local ISO date-time with seconds, such as 2026-09-14T09:00:00. UNTIL: UTC ISO with Z. DTSTART must itself match the selected recurrence.
DST gaps use the pre-transition offset and show the shifted clock time; overlaps use the first occurrence. Invalid calendar dates are skipped.
Calendar frequencies only. 8,192 UTF-16 units; 100 exclusions; COUNT up to 10,000. Preview: 1–1,000 dates, ten years from DTSTART, through year 2099; five-second timeout.
Input or options changed. Refresh the results before exporting.
Use the RRULE generator when a meeting repeats every other week, an invoice falls on the last day of each month, or one appointment must be excluded. A valid-looking string is only part of the check: the start date, calendar filters and timezone must agree.
The RRULE parser accepts an iCalendar recurrence rule and optional date properties. Use the RRULE tester to inspect next occurrences before putting the fragment into an event. The preview starts at DTSTART, not at the current date.
The RRULE generator keeps these two intentions distinct. BYMONTHDAY=31 skips months without a 31st; BYMONTHDAY=-1 selects each month’s last day. Choosing the negative index makes February visible instead of silently changing the meaning of a fixed date.
EXDATE removes an exact occurrence after COUNT has limited the original series. If a five-occurrence schedule loses one date, four remain. The report shows the removal count and preserves the original end condition.
Start with a preset to get both the rule and a matching DTSTART. Changing a Monday rule to a second Tuesday requires changing the start too. Form and parsing modes retain separate input; changing modes does not overwrite either draft.
The start and form exclusions are local ISO date-times with seconds in the selected IANA zone. UNTIL is an absolute UTC date-time ending in Z. An embedded DTSTART overrides the fallback fields and is shown in the report.
Automatic calculation waits 350 ms; switch it off for Analyze or Ctrl/Cmd+Enter. Edits disable stale exports. More dates appear in batches of 50; copied reports and JSON contain the entire calculated preview. Example and Clear require review, with one-step undo.
INTERVAL advances the chosen frequency. BYDAY uses weekday codes or ordinal weekdays for monthly and yearly rules. WKST determines the week boundary, which can change an every-other-week pattern.
BYMONTH selects months; positive and negative BYMONTHDAY values select dates. BYSETPOS then selects positions from the frequency period’s candidates. Combining weekdays with position -1 finds the last Monday-to-Friday date, without assuming a holiday calendar.
A bare rule uses the selected start and zone. DTSTART lines must use UTC Z or TZID. The canonical fragment records that effective context, so copying a rule does not quietly lose the calendar’s starting point.
Each row includes the requested clock time when a gap shifts it, the resolved local time, its offset and UTC. A weekly morning meeting normally retains its local hour while its UTC time changes across daylight saving transitions.
COUNT limits original occurrences, including the matching start, before exclusions. UNTIL is an inclusive UTC cutoff. Select one: a finite count can produce fewer visible dates after EXDATE removes occurrences.
Use the fragment in software that accepts recurrence properties. A complete calendar event additionally needs event metadata such as UID and DTSTAMP; this tool neither supplies those fields nor validates a full ICS file.
EXDATE is a separate property, not a parameter inside RRULE. Enter the exact local time in the form or supply an EXDATE line with Z or TZID. The normalized output writes exclusions as UTC instants.
The chosen IANA zone controls each local date. A regular 09:00 meeting keeps that hour across the transition. A nonexistent time uses the pre-transition offset; an ambiguous repeated time chooses its first occurrence.
BYMONTHDAY=31 skips shorter months. Use BYMONTHDAY=-1 for the last day of every month. The tool does not automatically replace the 31st with a nearby day, because that would change the rule.
Unexpected schedule?
Use the RRULE generator to compare the effective start, exclusions and resolved dates together. Export the fragment only after the preview reflects the recurrence you intend.