Hi Steven
There is a option in the Planner section of your Settings where you can choose to 'Replan From' either the 'Next Due Date' or the 'Last Done Date'. The difference is best explained with an example...
Let's say you had a monthly job due on the 1st of the month, but you do this job two days late on the 3rd. If you plan from the due date, the job will be kept to the original schedule, so will be planned for the 1st of the next month, but if you plan from the last done date it would be shifted to the 3rd next month.
Hope this makes sense and relates to original question, otherwise I have probably only added to the confusion!
Thanks
Chris.