install libical (/usr/ports/devel/libical)
run gen.sh in this directory
find ical_occurrences ruby extension in ./

irb
>> require 'ical_occurrences'
occ_array = Ical_occurrences::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.