My DAV server has hacks for Microsoft bugs and misfeatures, hacks for older Apple clients, hacks for Konqueror of all things, because I tested against it.
And our current CalDAV code has just inherited two new hacks this week to work around weird bugs in shit that Google Calendar has been serving up:
* years with only two digits or two leading zeros rather than 20xx.
* unquoted TZNAMEs with :s in them.
At least events from year "0012" are allegedly legitimate parsable ISO8601 times, and the events from year "12" are at least legitimate VCALENDAR. The broken TZNAMES parse legitimately, but
DTSTART;TZNAME=GMT+10:00:20120101T01010101 needs to be fixed up pre-parse.
Welcome to interoperability, where liberal in what you accept is the only choice when your communication partner is much bigger.