Date Difference – Month and Date Only

How can I use the Date Diff function but restrict the difference to just month and day?

For example:

(function(){
var obj = DATEDIFF(fieldname2, fieldname3, ‘mm/dd/yyyy’, ‘d’);

return (obj[‘days’]);

})()

I want to find the difference in days between the month and day not including the years.

I tried:

(function(){
var obj = DATEDIFF(fieldname2, fieldname3, ‘mm/dd’, ‘d’);

return (obj[‘days’]);

})()

but it didn’t work.