I've asked essentially the same question here:
I've yet to find a definitive answer, but lack of response suggests it doesn't work that way.Black MacBook (2008) 2.4 GHz 2GB, Mac OS X (10.5.6), iPod 60GB 5thGen/iPod Touch 32GB 2.2/Windows XP/"See that which cannot be seen"
Doesn't work that way although it would be nice to customise like you suggest.
As far as I can see, all the working day delimiters do is change the colour of the area bounded by those times in the display.20" iMac Intel Core 2 - 2.16GHz, 2Gb RAM, Mac OS X (10.5.6)
iCal's plist file has the day's start and end time as two keys. There is not a way to set these in the user interface. I think this is because if they are changed in a certain way without changing other preferences it can make iCal act rather strangely.
I have written an Applescript that sets the visible range in iCal to what you want. It also sets some other preferences so you should see what you expect to. It restarts iCal if it is running.
<Edited by Host>Mac OS X (10.5.6)