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RRULE explainer
FREQ=MONTHLY;BYDAY=-1FR is a sentence written for a parser, not a person. Paste one in and get the sentence back in English, with the dates it actually produces.
in English
Every month on the last Friday
the next occurrences, from today
{
"freq": "MONTHLY",
"byDay": [
{
"weekday": "FR",
"ordinal": -1
}
]
}where this comes from
It is one function out of a calendar.
This page runs parseRRule(), describeRRule() and expandRecurrence() from kloq — the same code that ships in kloq, not a re-implementation. Every interaction in the calendar is built to this standard, which is the whole pitch.