Hi Bernie
I use the Time Master ll from Calculated Industries Inc.
It adds up hours, minutes and seconds.
It is really a production timer which is hand held and also can be used to count down and count up time with all sorts of other useful functions. It is useful when you are wanting to pre-fade and come out of a live broadcast exactly on time - driving a real audio desk (do such things still exist) without all these new fangled computer thingies ...
I am sure there will be some newer software versions of production timers around now.
I have had my Time Master ll for over 10 years now and I'm not too sure if it is still on sale.
Charlie