# `RecurrenceRule`

When something recurs (RFC 5545): its first occurrence as a local time in its zone, a line feed, then its rule: `DTSTART;TZID=Europe/London:20261026T093000\nRRULE:FREQ=WEEKLY;BYDAY=MO`. `temporal.occurrences` gives the next ones.

[Time and calendar](types.md#time-and-calendar); not ordered; since Jadpo 0.1.0.

## Make one

Build one from its text, `RecurrenceRule("DTSTART;TZID=Europe/London:20261026T093000
RRULE:FREQ=WEEKLY;BYDAY=MO")`, which checks it: text that isn't one breaks `format`.

`text.parse(raw, as: RecurrenceRule)` reads one from its text; text that isn't one at all fails `malformed`, such as `FREQ=WEEKLY;BYDAY=MO` or `""`.

## Text, JSON and storage

`text.from` writes its canonical text, the one form `text.parse` reads back. JSON sends its canonical text as a string.

| Text | JSON |
|---|---|
| `DTSTART;TZID=Europe/London:20261026T093000\nRRULE:FREQ=WEEKLY;BYDAY=MO` | `"DTSTART;TZID=Europe/London:20261026T093000\nRRULE:FREQ=WEEKLY;BYDAY=MO"` |
| `DTSTART;TZID=America/New_York:20260310T090000\nRRULE:FREQ=MONTHLY;BYDAY=2TU;COUNT=12` | `"DTSTART;TZID=America/New_York:20260310T090000\nRRULE:FREQ=MONTHLY;BYDAY=2TU;COUNT=12"` |
| `DTSTART;TZID=Europe/London:20260130T170000\nRRULE:FREQ=MONTHLY;BYDAY=-1FR;UNTIL=20261231T235959Z` | `"DTSTART;TZID=Europe/London:20260130T170000\nRRULE:FREQ=MONTHLY;BYDAY=-1FR;UNTIL=20261231T235959Z"` |
| `DTSTART;VALUE=DATE:20240229\nRRULE:FREQ=YEARLY` | `"DTSTART;VALUE=DATE:20240229\nRRULE:FREQ=YEARLY"` |

It is stored in one column named for the field, of type `text`. `DTSTART;TZID=Europe/London:20260130T170000\nRRULE:FREQ=MONTHLY;BYDAY=-1FR;UNTIL=20261231T235959Z` is stored as `E'DTSTART;TZID=Europe/London:20260130T170000\nRRULE:FREQ=MONTHLY;BYDAY=-1FR;UNTIL=20261231T235959Z'`.

## Shown to people

`text.format(value, locale:)` shows it to people in their locale.

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `"DTSTART;TZID=Europe/London:20261026T093000\nRRULE:FREQ=WEEKLY;BYDAY=MO"` | `"DTSTART;TZID=Europe/London:20261026T093000\nRRULE:FREQ=WEEKLY;BYDAY=MO"` |
| `"DTSTART;TZID=America/New_York:20260310T090000\nRRULE:FREQ=MONTHLY;BYDAY=2TU;COUNT=12"` | `"DTSTART;TZID=America/New_York:20260310T090000\nRRULE:FREQ=MONTHLY;BYDAY=2TU;COUNT=12"` |
| `"DTSTART;TZID=Europe/London:20260130T170000\nRRULE:FREQ=MONTHLY;BYDAY=-1FR;UNTIL=20261231T235959Z"` | `"DTSTART;TZID=Europe/London:20260130T170000\nRRULE:FREQ=MONTHLY;BYDAY=-1FR;UNTIL=20261231T235959Z"` |
| `"DTSTART;VALUE=DATE:20240229\nRRULE:FREQ=YEARLY"` | `"DTSTART;VALUE=DATE:20240229\nRRULE:FREQ=YEARLY"` |

## Operations

- `temporal.occurrences(rule: RecurrenceRule, from: T, count: Int) -> List<ZonedTime>`: The next `count` occurrences of `rule` at or after `from`, in its zone (RFC 5545): fewer when the rule ends first; `count` is from 1 to 1000, so an unbounded rule is never expanded without a bound. From a date, the days they fall on, `List<CalendarDate>`, which a rule on dates gives; from an `Instant`, a timed rule's local times.

Like every standard type, it also has the contract's own operations: `text.parse`, `text.from`, `text.format`, `json.read`, `json.write` and `==`. It isn't ordered, so `<` and sorting don't take it. [Every standard type meets one contract](../topics/every-standard-type-meets-one-contract.md) says how they work.