问题解决后，日期应为“y-m-d”格式，而不是“d-m-y”，如下所示：

if (isset( $_REQUEST['signupnbtn'] ) ) {
    $age = mysqli_real_escape_string($con,$_REQUEST['ageinput']);
    $discord = mysqli_real_escape_string($con,$_REQUEST['discordinput']);
    $email = mysqli_real_escape_string($con,$_REQUEST['emailinput']);
    $tmp = mysqli_real_escape_string($con,$_REQUEST['tmpinput']);
    $steam = mysqli_real_escape_string($con,$_REQUEST['steaminput']);
    $datum = date("Y-m-d");
    $errorcode = 0;
    $q = "INSERT INTO `admissions` (age, discord, email, tmp, steam, datum)
    VALUES ('$age', '$discord', '$email', '$tmp', '$steam', '$datum')";
    $query2 = "SELECT email FROM `admissions` WHERE email='$email'";
    $sql = mysqli_query($con,$query2);
    $countrows = mysqli_num_rows($sql);
    if($countrows >= 1){
      $errorcode = 1;
    }else {
      $result = mysqli_query($con,$q);
    }
      if ($result) {
        $errorcode = 4;
      }
  }

$q = "INSERT INTO admissions (age, discord, email, tmp, steam, datum) VALUES ('$age', '$discord', '$email', '$tmp', '$steam', '$datum')";