This works in an assignments report column:
That being said, I have not been able to successfully use text based expressions (IF, CONCAT, etc) on date/time calcs in a form. If you even DID get it to work, it would not honor the standard time zone rules as all text based calcs use the default time zone when referencing dates.
This doesn't say invalid expression but it still doesn't work:
IF(Planned Completion Date=Actual Completion Date,"yes","no")
I suspect it's because the predecessor date is a reference and form calc won't even entertain that.
Because Workfront hasn't remedied this, I would suggest a report-based calculation rather than form-based.
But if you do get anything to work, I'd love to hear how it's done.