Jon,

I agree with you that there should be no problem with your table.

The only reason for using calculated date(part)s would be if you need to use relative date ranges ie Previous Quarter, This Quarter or whatever.

You might like to take a look at my date ranges function:

http://www.sqlservercentral.com/scripts/contributions/864.asp

(There is a new improved version waiting for approval - Mail me directly if you are interested)

It only goes down to the level of a whole day, but there is no reason not to expand it down to the quarter hour level you are interested in.