Use gform_pre_submission to combine two fields

Hello,
I separated the birthday field into two text fields for MailChimp. These fields are: id 83, birth_month_day, and id 81, birthyear.

Now, i'd like to use the gform_pre_submission hook to automatically show a birthday field that results in 83 plus 81. i.e. field id 85 shows mm/dd/yyyy. I am not a coder, but am making feeble attempts to do this. 2 of 3 forms have these age fields, the form ids are 7 and 11.

Here is the gform_pre_submission example with my field ids.

<?php
add_action("gform_pre_submission", "birthday");
function birthday(){
    $_POST["input_85"]  = "input_81"&"/"&"input_83";
}
?>

and—in the new birthday field, id 85, I have populate dynamically with the parameter, birthday. It is returning ) in the birthday field.