if you insist on using the 2018-08-31 format, you can add the locale option when initializing datepicker, as shown below

$(function() {
  $('input[name="daterange"]').daterangepicker({
    opens: 'left',
    locale: {
      format: 'YYYY-MM-DD'
    }
  }, function(start, end, label) {
    console.log("A new date selection was made: " + start.format('YYYY-MM-DD') + ' to ' + end.format('YYYY-MM-DD'));
  });
});
</script>

1. date and time problems, check the required format first;
2. hard debugging. If PHP doesn't work, HTML directly writes it, and HTML doesn't want to copy the original code in the Demo given by the plug-in's official website, and then change to a version that can run normally, and then check where the code that can run in the end is different from the one written before.