Codice commento: https://skillsandmore.org/usare-javascript-wordpress/#comment-721

new-script.js

jQuery(function() {
  // Check the date range, 86400000 is the number of milliseconds in one day

    $("#start_date").datepicker({ 
        dateFormat: "dd/mm/yy",
        changeMonth: true,
        changeYear: true,
        onSelect: function(selected) {          
           $("#end_date").datepicker("option","minDate", selected)
        }
    });

    $("#end_date").datepicker({ 
        dateFormat: "dd/mm/yy",
        changeMonth: true,
        changeYear: true,
        minDate: $("#start_date").val(),
        onSelect: function(selected) {
          $("#start_date").datepicker("option","maxDate", selected)
        }

    }); 

});

old-script.js

$(function() {
  // Check the date range, 86400000 is the number of milliseconds in one day

    $("#start_date").datepicker({ 
        dateFormat: "dd/mm/yy",
        changeMonth: true,
        changeYear: true,
        onSelect: function(selected) {          
           $("#end_date").datepicker("option","minDate", selected)
        }
    });

    $("#end_date").datepicker({ 
        dateFormat: "dd/mm/yy",
        changeMonth: true,
        changeYear: true,
        minDate: $("#start_date").val(),
        onSelect: function(selected) {
          $("#start_date").datepicker("option","maxDate", selected)
        }

    }); 

});