Add fields on the Project and Task to track additional dates. The following dates are: Original Contractual Start Date; Original Contractual End Date; Revised Contractual Start Date; Revised Contractual End Date, Project Actual Start Date; Project Actual End Date (system update automatically when status changed to Active and when Project status changed to Completed).
Also, have Change Order with additional time automatically extend the Revised Date.
This is useful for clients to look at to determine how they are doing against the time line. Also, helpful when potential liquidated damages exist on a project.