I was hoping to create a quick sign-up list for volunteering at an
event. The options are 3-4 hour shifts between 5-10PM on a Saturday
or 2-10PM on a Sunday.

Initially, the options were similar for both days, so I created a
choice for arrival time (2,3,4,5,6,7) and a choice for # of hours
(3,4,5). I then created a "shift" column using this formula:
=[Arrival Time (PM)]&"PM to "&[Arrival Time (PM)]+[# of Hours You
Can Work]&"PM"

This was cool and worked pretty well, but now that it's only 5-10PM
for one of the days, I was trying some more sophisticated formulas.
I was trying to block them from signing up for 2,3, or 4 if they
choose Saturday.

Anyway, this is totally silly, but I really want to learn as much as
I can about the calculated column options. I got giddy with delight
when I set up the simple one, so please help me do something even
better!
I've been reading through the MS help files on
calculated columns, but I'm not finding exactly what I need. Maybe
it doesn't exist unless you write some code somewhere...