Include Submitted Date in Array

Here a snippet of PHP code I have in a custom page:

$form_id = 20;
$submissions = Ninja_Forms()->form( $form_id )->get_subs();

if ( is_array( $submissions ) && count( $submissions ) > 0 ) {
	foreach($submissions as $submission) {
	$submission_values = $submission->get_field_values();
	$firstname = $submission_values['firstname_1597434182503'];
	$lastname = $submission_values['lastname_1597434183111'];
	$email = $submission_values['email_1597434203863'];
	$phone = $submission_values['phone_1597434204437'];
				
	{
		echo '<tr><td>' . $firstname . '</td><td>' . $lastname . '</td><td>' . $email . '</td><td>' . $phone . '</td>';
	}
}

Is there a way to also include the date submitted? I can’t seem to find the key when I print the array?