The GetfieldSelections() returns a string describing your selection, it does not return an array of values. It will be hard to format each of these, there is also a default limit to the amount of selection listed before it starts to return "X out of 1234".
I suggest you either create a field with the format 'MMMDD' and do the selections on that field. Or if the check for selection not important, but you just want a list of available values then you could use concat() instead. Maybe like this?
=concat(Distinct Date([orderdate],'MMMDD'), ', ')
