Ajax/js function does not work

Question

It follows below my code, with the function and the field. When testing does not work anything, I do not know what is wrong. I did this function to check if such a period already exists in the bank. But I don’t even know how to check if the php file is receiving the data or if the result is coming back and I’m not displaying correctly.

Thanks in advance for the suggestions.

function verifica(){                  
      var id  = document.getElementsByName("id")[0].value;
      var dt1 = document.getElementsByName("datahora_inicial")[0].value;
      var dt2 = document.getElementsByName("date")[0].value;
      var parametros = {
              method: "GET"
      };
      fetch("php/verifica.php?id=" + id + "&dt1=" + dt1 + "&dt2=" + dt2, parametros).then(function(resposta) {
          return resposta.json();
      }).then(function(retorno){
          console.log(retorno)
          if (retorno == 1) {
            document.getElementById("resultado").value = "Período indisponível! Altere-o.";
            document.getElementById("resultado").style.display = 'inline-block';
            document.getElementById("link").style.display = 'inline-block';
          }else{
            document.getElementById("resultado").style.display = 'none';
          }
      });
  }




<label class="col-sm-2 col-sm-2 control-label">Fim</label>
<div class="col-md-5">
  <input type="text" class="form-control round-input" name="date" required="required" id="dtfim" onchange="verifica();" value="" disabled="disabled">
   <input type="text" class="form-control round-input" id="resultado" disabled="disabled" style="display: none; width: 60px;"><a style="display: none;" href='link.php' id="link">Link</a>                                                            
</div>

php:

header('Content-Type: json/application'); 
$id  = $_GET['id'];
$dt1 = $_GET['dt1'];
$dt1 = date('Y-m-d', strtotime($dt1));

$dt2 = $_GET['dt2'];
$dt2 = date('Y-m-d', strtotime($dt2));

$select    = "SELECT * FROM lista WHERE id = '$id' and start <= '$dt1' and end >= '$dt2'";
$result    = mysqli_query($conexao, $select);

if ((mysqli_num_rows($result) > 0)){
    $mostrar = 1;
    echo json_encode($mostrar); 
}else{
    $mostrar = 2;
    echo json_encode($mostrar);
}