libical

First of all make sure you have libical installed. Without it the gem won’t install.

$ gem sources -a http://gems.github.com (if you haven't already)
$ sudo gem install erdgeist-chaos_calendar

If the install fails because of missing header files try:

$ sudo gem install erdgeist-chaos_calendar -- --with-ical-include=/path/to/include/dir

on Mac OS X

$ sudo gem install erdgeist-chaos_calendar -- --with-ical-include=/opt/local/include

on FreeBSD:

$ sudo gem install erdgeist-chaos_calendar -- --with-ical-include=/usr/local/include

irb
>> require 'chaos_calendar'
occ_array = ChaosCalendar::occurrences( "20000101T230000", "20091231T230000", "FREQ=DAILY;COUNT=1200" )

Be aware that the array is in UTC and will be converted to your local timezone before it is used.