how to write a callback function, after the datapicker date selection?

Question

I am using datapicker on a field to pop out calender. after the selection of date by using this datapicker i want to execute a callback function, in that function i want to set this selected date value to another field.

this is my currrent code

<%= text_field_tag "from_date",Date.today.beginning_of_month.strftime("%d-%b-%Y") , :class=>"form-control from_picker", :id => "start",'data-behaviour' => 'datepicker',:placeholder =>"From" %>


<script type="text/javascript">
  $.fn.datepicker.defaults.format = "dd-M-yyyy";

  $(document).on("focus", "[data-behaviour~='datepicker']", function(e){
    $(this).datepicker({"autoclose": true,
                         onSelect: function(selectedDate) {
                           alert(selectedDate);
                           var from_picker = $(".from_picker").val();
                           $("#d_start").val(from_picker.toString());
                         }
    });
  });
</script>


<%= text_field_tag "set_date",Date.today.beginning_of_month.strftime("%d-%b-%Y") , :class=>"form-control", :id => "d_start",'data-behaviour' => 'datepicker',:placeholder =>"From" %>

the datapicker is working fine. am able to select the date. but the callback is not getting executed, because of that i am unable set this value to other field.