Once a schedule is underway and planned tasks are actually executed, the schedule needs to be updated to reflect what really happened. Needless to say, what really happens is usually different from what was planned! And reconstituting a picture of reality down to the tiniest details is not always easy or practical.
It is impossible to automatically update a schedule often enough for it to always be a true reflection of reality at any given moment. We have to periodically enter information about what has really happened, for instance once a week, or at best once a day.
If 'Slip Tasks to As-Of Date' (in Options, Calculations) is unchecked, all future tasks will be allowed to slide back in time to either the schedule start date, or to where a predecessor dependency controls it, whichever is later. So it makes sense to leave 'Slip Tasks to As-Of Date' unchecked when entering historical (actual) information, and then make adjustments to future tasks with Slip Tasks to As-Of checked.
We recommend the following sequence when updating the schedule.
1. Uncheck Slip Tasks to As-Of Date, and Recalculation to "Manual" in Options, Calculations.
2. If you are using a "manual" As-Of Date (instead of setting it to "Today"), change the As-Of Date to the date for which you are updating. Click OK on the Options, Calculations form.
3. Recalculate the schedule, if necessary.
4. Update the appropriate task information by updating each task manually, or (if all task dates to the left of the As-Of Date line are correct) by using Options, Update Percent Complete.
5. Return Slip Tasks to As-Of Date to checked in Options, Calculations, if desired. The next time the schedule is recalculated, this will cause tasks that haven't started yet (in other words with Percent Complete = 0), but whose start dates are to the left of the As-Of Date, to be rescheduled to start no earlier than the As-Of Date.