I've had a few calls over the last couple weeks with folks who'd like to use their histories in GoldMine to generate billing for clients. I actually do this myself every week. Where the challenge lies is in the fact that the "Duration" field on a History item reads like this: "00:30:00" (30 minutes).
This makes any totaling of these durations (say, on a report or within a query) difficult to say the least.
I find it much easier to enter in "Decimal" times for my histories. So, instead of using "00:30:00" for a half hour, I use ".5" Note that the Duration field is a simple text field, and will accept almost any input you give it. So my 01:45:00 becomes 1.75 and so on.
This way, any totaling you want to do becomes easy. It is possible to "parse" out the HH:MM:SS duration value, but you'll spend more time doing that than working on the report itself...
This also brings up a good concept; whenever possible, structure your data entry model to make reporting easier.
Try it, and have fun!