Uncaught TypeError: $(...).datepicker is not a function

Uncaught TypeError: $(...).datepicker is not a function

hi
here is my code. I want to choose a date and the other will automatically give me 2months later date. its shows me this error "Uncaught TypeError: $(...).datepicker is not a function"
  2. <!DOCTYPE html>
  3. <html>
  4. <head>
  5. <title>Date Choose Option</title>
  6. <script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>

  8. </head>
  9. <body>
  10. <input type="text" class="firstcal">
  11. <input type="text" class="secondcal">
  12. <script>
  13. $(function() {
  14.     $(".firstcal").datepicker({
  15.         dateFormat: "mm/dd/yy",
  16.         onSelect: function(dateText, instance) {
  17.             date = $.datepicker.parseDate(instance.settings.dateFormat, dateText, instance.settings);
  18.             date.setMonth(date.getMonth() + 2);
  19.             $(".secondcal").datepicker("setDate", date);
  20.         }
  21.     });
  22.     $(".secondcal").datepicker({
  23.         dateFormat: "mm/dd/yy"
  24.     });
  25. });
  26. </script>

  28. </body>
  29. </html>
please help me.
thank you