Courses can either be moved on a specific date, or a certain period of time after they have begun.
For example, if a category has a schedule set to archive courses on June 1, 2009, then all courses in that category (with the exception of those that have their own individual schedules) will be archived on June 1, 2009.
However, if a category has a schedule set to archive courses 3 months after they begin, then each course in that category (with the exception of those that have their own individual schedules) will be moved 3 months after it began.