MYSQL INSERT INTO Produces An Error: You have an error in your SQL syntax

I am trying to make a simple shoutbox for a school project.

Everything seems to be working fine, except when i try and send a message. My sql query is simple, but seems to not be working for some reason.

<?php
session_start();
require_once("includes/connect.db.php");

$sql = "SELECT * FROM shoutbox";
$result = mysql_query($sql);

echo '<table border=1>';
while($rows = mysql_fetch_assoc($result)){
    $sb_username = $rows['username'];
    $sb_message = $rows['message'];
    $sb_sent_time = $rows['sent_time'];
    echo '<tr><td>' . $sb_username . ': </td><td>' . $sb_message;
}
echo '</table>';
?>
<form method=post action=shoutbox.php>
    <input type=text name="message">
    <input type=submit value="Send!">
</form>

<?php
if(isset($_POST['message'])){
    $date = time();
    $message = mysql_real_escape_string(htmlentities($_POST['message']));
    $username = $_SESSION['user_name'];
    $sql = "INSERT INTO shoutbox ('username', 'message', 'time_sent') VALUES ('$username', '$message', '$date')";
    mysql_query($sql) or die(mysql_error());
}

?>

Produces the following error: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ''username', 'message', 'time_sent') VALUES ('c4sper', 'hello', '1461107151')' at line 1

Replace the following line in your code with this new one:

 $sql = "INSERT INTO shoutbox (`username`, `message`, `time_sent`) VALUES ('$username', '$message', '$date')";

Note : Use `` (Backticks) instead of '' (Quotes) around your table column's (fields) names in your INSERT query.

For detailed guidance,Take a look at :

When to use single quotes, double quotes, and backticks in MySQL