rrule.js

rrule.js is the JS port of python-dateutil's rrule module. It parses and serializes RRULE strings ("FREQ=WEEKLY;BYDAY=MO,WE,FR;UNTIL=20261231T235959Z"), expands them into concrete occurrence lists, and supports all the iCalendar RFC 5545 features. 1.5k stars. Used in FullCalendar, various calendar widgets, and basically every "subscription" feature in JS apps. The original maintainer (Jakub Roztocil) passed away in 2022, and the project is on the maintainers-wanted list. The community has been doing small maintenance, but the issue backlog is growing. A rescue here would mean committing to the RFC spec, which is one of the more complex standards to implement correctly.