Custom function validate_date creates error

Question

I'm trying to create a custom function to validate the date & I'm following a tutorial, the first code is before the changes, the second is after. After I try and run the changed code I get an undefined variable: date error related to line if ($time = strtotime($date))

if($_SERVER['REQUEST_METHOD'] === 'POST'){
    $date = trim($_POST['date']);
    $email = trim($_POST['email']);
    $description = trim($_POST['desc']);

        if (!empty($date) && !empty($email) && !empty($description)) {

            ***if ($time = strtotime($date)) {
                echo '<p>Date:'. date('F jS Y' ,$time).'</p>';***
            }
                if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
            echo "<p>Email: $email</p>";
                }
            echo '<p>' . strip_tags($description) . '</p>';
    }
}

---

if($_SERVER['REQUEST_METHOD'] === 'POST'){
    $date = trim($_POST['date']);
    $email = trim($_POST['email']);
    $description = trim($_POST['desc']);

***function validate_date($date_string)
    {
        if ($time = strtotime($date)) {
            return date('F jS Y', $time);
        } else {
            return $date_string . ' does not look valid.';***
        }
    }
        if (!empty($date) && !empty($email) && !empty($description)) {

        ***echo validate_date($date);***
            if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
        echo "<p>Email: $email</p>";
            }
        echo '<p>' . strip_tags($description) . '</p>';
}

}