RRULE Builder & Explainer

How RRULE differs from cron

Cron describes absolute clock times a job should fire at ("9am, every weekday"). RRULE describes a pattern relative to a starting event's own date and time, defined in the iCalendar standard (RFC 5545) — it's built into the DTSTART/RRULE pair inside a VEVENT block, not a standalone schedule. That's why calendar apps use RRULE and job schedulers use cron, even though both ultimately answer "when does this happen again."

Where this fits with a full calendar file

To turn a generated RRULE into an actual importable event, add the line as RRULE:... directly beneath the DTSTART line inside the block from the ICS Calendar File Generator.