I'm a bit confused between field codes {F9, a-F9, & c-F9} and field controls.

I've tried field code several times and rarely get them to work. I tried to calculate the end of the loan term by using the VBA function DateAdd("M", StartDate, nper), where StartDate and nper are numeric Text Form Fields. Can field codes reference these? I tried something like:
{ =DATAADD("m", {REF StartDate }, {REF nper} ) \@ "MM/dd/yyyy" }

But get
!Syntax Error, (

Sooo, instead I did everthing with Legacy Text Form Fields and the Exit field.
What I was initially asking, is there a way in the exit sub to tell if the formfield was actually changed?

My hack was to create Static variables then compare that static var value to the formfield value:
ActiveDocument.FormFields("StartDate").Result
if different, I update other FormFields, then before leaving, copy the current formfield value into the static variable.

This is mostly working but somewhere the static vars are re-initialized (Using the debug.print command)

(My previous message was trying to use field codes instead of formfields exit sub
Ken