It is on this page
https://formidableforms.com/knowledgebase/javascript-examples/#kb-calculate-final-date
 
i have done all the changes but it is not working for me
that is my code All field ids are correct. check it 3 times
<script type="text/javascript">
jQuery(document).ready(function($){
$('#field_siq5u, #field_1jta1').change(function(){

var addDays = $("#field_siq5u").val();
var dateOneString = $("#field_1jta1").val();
if ( addDays === '' || dateOneString === '' ) {
return;
}
var finalDate = $("#field_1jta1").datepicker( 'getDate' );
finalDate.setDate(finalDate.getDate() + parseInt( addDays ) );
$("#field_yeatz").focusin();
$("#field_yeatz").datepicker("setDate", finalDate);
$("#field_yeatz").change();
});
});
</script>