I have to admit, the Timer functionality in GoldMine is rarely used. But for those of you administrating a Cold Calling salesforce it can help in long term analysis of calls. Which is to say the Timer can be configured to automatically fill the Duration box when completing an activity.
To enable this (on a per user-basis) go to Tools | Options | Schedule Tab. Check the box labeled "Start Timer when Completing Activities". Ok your way out.
Now, go to Complete | Unscheduled Call | Incoming Call. You'll notice now that when you complete items the Timer will start running (in the lower right-hand corner of GoldMine). The call taker can then simply use the Notes area during the call. The timer will continue to run as long as the Complete window is open. Hitting Ok stops the timer and fills in the Duration.
Pro Tip : The CONTHIST.DURATION field is stored as a STRING. Mathematical analysis (on the back end) of the CONTHIST.DURATION field will require you to convert the data type from a STRING to a NUMBER.