You don't need a case statement, for this an if statement is easier.
If you just want to display the current date on your layout then you would just need a calculation
(Make sure the reture type is date)
then click Storage Options and then click "Do not store calcualation results, recalculate as needed".
This can be simplified even more:
If(IsEmpty(DateA);Get(CurrentDate) ; DateA - 134 )