Yes, use a time stamp field. Either enter the data into a timestamp field to begin with or use a calculatin field that combines the two into a time stamp field. And a time interval that crosses midnight will still calculate correctly when you use a time stamp field.
Like time fields, subtracting on TS field from another computes the elapsed time in seconds. You can use such a caclualtion with either a number or a time return type.
Thanks Phil, I prefer the easier ones like this ;)