Is there a way to express the following in iCalendar? (possibly using RRULEs?)
Between 9am and 5pm every day except Sundays when it is between 10am and 4pm.
Can I create multiple RRULEs? Which ones have precedence?
iCalendar does not have a notion of events with a hole in the middle of the duration. So even without talking about recurring event, you can not create with a single event something like from 9AM to 10AM and from 4PM to 5 PM
As a consequence, you need to actually create 3 events:
DTSTART:2013xxxxT090000
DURATION:PT1H
RRULE:FREQ=DAILY
DTSTART:2013xxxxT100000
DURATION:PT6H
RRULE:FREQ=DAILY;BYDAY=MO,TU,WE,TH,FR,SA
DTSTART:2013xxxxT160000
DURATION:PT1H
RRULE:FREQ=DAILY
As far as having multiple RRULE, this was supported in RFC2445 but it has been deprecated in RFC5545 (just like EXRULE) due to lack of support and complexity.