Ciao benvenuto,
usa i tag code per il codice.

codice:
function check_booking_form() {
var book_from, book_to;
book_from = $("#start_date").val();
//book_to = $("#end_date").val();

if (book_from === '' ) {
$('#booking_form_request_mess').empty().addClass(' book_not_available').show().append(property_vars.p lsfill);
return false;
} else {
return true;
}
}