One technique would be to create a Task level "Planned Percent Complete" calculated parameter that computes the percent complete based upon the current date using the Planned Start Date and Planned Completion Date (e.g. 0% if before, 100% if after, somewhere in between if in between, linearly). You could then reported the Task Percent Complete next to that Planned Percent Complete, noting that you'll need to recalc the latter every day to keep it current, either manually, or using (for example) our Recalc Parameters solution (e.g. every night). You could then update the former with the latter to "force" the Task to stay on track, either manually, or using (for example) our Excel Updater solution to do so on mass. To then ensure Timelines are then adjusted accordingly (e.g. driving the Project Percent Complete), you could then recalculate timelines manually, or using our Force Timeline Recalculation solution (e.g. every night).
Once you've proven the theory (and value), you could then automate the whole shebang (e.g. using Fusion, API, our UberCalc solution, etc.)