According to the [document](https://datatracker.ietf.org/doc/html/rfc5545), 5545 obsoletes 2445 since 2009. The README.md states that, > with a goal of being fully compliant with [iCalendar spec (RFC2445)](https://tools.ietf.org/html/rfc2445) I stumble upon this while trying to use this gem (RFC2445) with [rrule in javascript](https://github.com/jakubroztocil/rrule) (RFC5545). I am not familiar with the RFC5545 or RFC2445.