Hi Seth,
This is possible to do with the following expression:
IF(ISBLANK({actualStartDate}),{plannedStartDate},{actualStartDate})
Or in a column on a report, you'd use the following:
displayname=Start
textmode=true
valueexpression=IF(ISBLANK({actualStartDate}),{plannedStartDate},{actualStartDate})
valueformat=HTML
Best Regards
Rich