The best way to schedule something in GoldMine is from the graphical calendar (Go To | Calendar). When you do it this way (as opposed to using the Schedule menu), the Date, Time and Duration are automatically filled using whatever swath of time you dragged out on the calendar.
1. Open the Calendar.
2. Using the Week or Day view, click-drag out a section of time.
3. GoldMine now prompts with the "Schedule a..." window. Please be aware that GoldMine automatically thinks this is an Appointment. You can feel free to change this to "Call" or "Next Action", or whatever may be applicable.
Notice that the scheduled item is linked to the Current Contact, that is, whomever we're looking at right now. If you need to relink to a different contact, simply click on the little "person" button to the right of the Contact name within the Schedule window.