Get ( current date ) works the same in your calculation as in a calculation field.
But your calculations I'm your example should be: month ( get ( currentdate ) ).
d = Get ( CurrentDate ) ;
Month ( d ) < 2 ; "Jan " & Year ( d ) ;
Month ( d ) < 6 ; "May " & Year ( d ) ;
Month ( d ) < 8 ; "Jul " & Year ( d ) ;
"Jan " & ( Year ( d ) + 1 )