Adding A Time Column To A Date Column

I have two columns in a table:
StartDate DateTime and StartTime DateTime.
The StartDate column holds a value such as 07/16/2004
The StartTime column holds a value such as 3:00:00 PM

I want to be able to add them in a stored procedure.
When I use StartDate + StartTime I get a date two days earlier than expected.
For example, instead of 7/16/2004 3:00:00 PM StartDate + StartTime returns
7/14/2004 3:00:00 PM.

Can anyone point out wht I'm doing wrong with this one?

Thanks,
lq