Adding time -- figuring time intervals

Hello,
I have a database that keeps track of user, batch, starttime and endtime. I am
subtracting starttime from endtime to find out how long each batch ran. That works
fine. I now want to add all of these results to find the total time of the batches per user
and I keep getting the error "expression to complicated to evaluate". Any help???

Try using datediff to return the difference as a number and then sum that