Two date calculation problems | WordPress.org

Hello;
I wrote the following code to calculate the 1st date and set the years automatically.

date1 : fieldname14 selected date

date2 : fieldname15 it gets 30/09/YEAR every year.

(function() {
  var current_year = YEAR(TODAY());
  var first_day_year="30/09/" + current_year;

  jQuery('#calculation-first_day_year').html(first_day_year);
  jQuery('.kriter1-aciklama').html('Başlangıç Hesaplama : ');
  jQuery('.kriter1-sonuc').html(GETDATETIMESTRING(first_day_year, 'dd/mm/yyyy'), 'dd/mm/yyyy');
})();

I get the selected date automatically in the 2nd date.

Master calculation code;

(function() {
  var ay = DATEDIFF(fieldname15, fieldname14, 'dd/mm/yyyy', 'y')['months'];

  jQuery('#calculation-ay').html(ay);
  jQuery('.kriter2-aciklama').html('ABC: ');
  jQuery('.kriter2-sonuc').html('<span style="color: orange;">' + ay + '</span>');

  return ay;
})();

where am i doing wrong?