Try defining the following fields:
1. Start (time)
2. End (time)
3. WorkTime (calculation = (end - start)/3600)
4. Date (date)
5. TimeInInterval (calculation = If (Date ≥ Get (CurrentDate) - 7 ; WorkTime ; "")
6. IntervalAvg (summary = Averge of TimeInInterval)
sadly, that did not work for me. i tried the formulas as you outlined them and the resulting average field is still only giving me the time for the last call. not sure where the glitch is. wish i could add the form here so folks could download it and have a look.