Dirk,
timespans and intervals are there for extreme flexibility.
This is a service package and not really meant to be called by end users.
Also, a big distinction is between and event and an activity. An event is an activity with a temporal value attached. IOW, Ride Mountain Bikes is an activity. Ride Mountain Bikes at 12 AM Mondays is an event (actually recurring).
This allows for repeating events of non-regular intervals (i.e Jan 4, Feb 13, May 9 and Dec 12) as well as regular intervals (i.e. every Friday), Allows for unknown start or ends (i.e. Starts at 3 PM, or ends Jan 13 2004). As well as many other options. acs-events is an extremely useful if underutilized service, and the problem with calendar is the data model for calendar not being written with the flexibility of acs-events (it was written originally in the LA AD office at the time the Java craze was beginning and about the time I left, so it was never really code reviewed or anything else, it was off to java land).
I hope that helps some with understanding acs-events, also Scott Meeks wrote the actual acs-events code so he may have some insights into the compromises he made to our original spec.
