The ical specification allows one to define some very interesting and complicated date recurrences. This table (adapted from rfc-archive.org/getrfc.php?rfc=5545) was very useful when I decided to rewrite the recurrence engine part of my plugins.
Sorry about the table width – got to get that theme css fixed!
| Yearly | Monthly | Weekly | Daily | Hourly | Minutely | Secondly | |
|---|---|---|---|---|---|---|---|
| BY MONTH (1 to 31, -1 to -31) | Expand | Limit | Limit | Limit | Limit | Limit | Limit |
| BY WEEK NO (1 to 53, -1 to 53) | Expand | invalid | invalid | invalid | invalid | invalid | invalid |
| BY YEAR DAY (1 to 366, -1 to -366) | Expand | invalid | invalid | invalid | Limit | Limit | Limit |
| BY MONTH DAY (1 to 31, -1 to -31) | Expand | Expand | invalid | Limit | Limit | Limit | Limit |
| BY DAY (MO, TU , WE, TH, FR, SA, SU) | Note 2 | Note 1 | Expand | Limit | Limit | Limit | Limit |
| BY HOUR (-23 to 0 to +23) | Expand | Expand | Expand | Expand | Limit | Limit | Limit |
| BY MINUTE (-59 to 0 to 59) | Expand | Expand | Expand | Expand | Expand | Limit | Limit |
| BY SECOND (-60 to 0 to 60) | Expand | Expand | Expand | Expand | Expand | Expand | Limit |
| BY SET POS (-366 to -1,1 to 366) | Limit | Limit | Limit | Limit | Limit | Limit | Limit |
The “By”‘s are applied in the order indicated. Limit means that the possible repeat dates are limited by the “BY” specification. Expand means that the possible repeat dates are expanded by the BY values. The start date will fill in any missing year, month, day, hour, minute or second values. See the RFC 5545 Specification for examples.
Note 1:
- Limit the dates if BYMONTHDAY is present;
- else special expand for monthly
Note2:
- Limit if BYYEARDAY or BYMONTHDAY is present; otherwise,
- special expand for WEEKLY if BYWEEKNO present; otherwise,
- special expand for MONTHLY if BYMONTH present; otherwise,
- special expand for YEARLY