[eluser]fesweb[/eluser]
The no end date thing has never come up with my particular needs, so I (thankfully) have never had to worry about it. Practically speaking, you can't really put an infinite number of instances into the db, so there would *have* to be another way to handle it.
One factor is the anticipated lifespan of the actual calendar you're building. The odds that your system will still be in use 10 years from now is fairly small, when you consider the rate of change in the available technology/services.
Also, you have to look at the actual data that you're working with. Are there actually things that repeat on a regular schedule without any managerial oversight for the next X years?