I found the answer on another post. The original post was answered by Magus069 and for those looking for this answer I'll share below.
The date field which you want it to be readOnly will need to have the following FormCalc script within the calculate event
if (DateTimeField1.rawValue <> null) then
Num2Date(Date2Num(DateTimeField1.rawValue,"YYYY-MM-DD") + 2, "YYYY-MM-DD")
else
null
endif