Hi Debi,
Have you tested out the new Journal Entry report type that was recently released (see here on Workfront One for more info)? I've only just started testing this functionality myself, but it seems pretty powerful and should be able to meet your requirement.
Try creating a new journal entry report with the following settings:
Groupings - Task>>Name
Filters - Journal Entry >> Field Name = plannedCompletionDate & Journal Entry >> Task ID = Is Not Blank
Columns (views) - 'Add Journal Entry >> New Date Value' and 'Journal Entry >> Old Date Value'
The report will now return all instances of a change to the planned completion date grouped by the task name, showing what the date was changed from and to (it also shows who made the change!)
Hope this helps!
Best Regards,
Rich.