I've using the DATEDIFF method to get the difference b/w two dates obtained using the date(YYYY:mm:DD HH:MM:ss) function but getting the output 0 every time, even when the difference is more than 1 day.

here's my code - 

```
if(isset($_GET['rt']))
{
$dateis=$_GET['rt'];
}
date_default_timezone_set('Asia/Kolkata');
$date = date('Y-m-d H:i:s');
if(mysqli_query($conn, "UPDATE issued_books SET Date_returned = '$date' WHERE Book_id = '$bk_id' AND Date_issued = '$dateis'"))
{
    $fine = sprintf("SELECT DATEDIFF('%s','%s') AS DAYS",
               '$date', '$dateis');
    $fine = $fine*10;
    mysqli_query($conn, "UPDATE member_db SET Fine_amt = (Fine_amt + '$fine') WHERE Member_id = '$mem'");
}

```

any kinda help would be really appreciated, thanks :)