public function update_schedule($old, $new)
 {
     if ($old == $new) {
         return;
     }
     // Schedule notifications event if notifications is active
     if (sm_is_notification_enabled()) {
         wp_clear_scheduled_hook('notification_event');
         wp_schedule_event(time(), sm_get_notification_recurrence(), 'notification_event');
     } else {
         wp_clear_scheduled_hook('notification_event');
     }
 }
<form method="post" action="options.php">
    <?php 
$settings = array('notifications' => sm_is_notification_enabled(), 'recurrence' => sm_get_notification_recurrence());
settings_fields('smartmockups_settings_notifications');
?>
    <table class="form-table">
        <tbody>
            <tr valign="top">
                <th scope="row" valign="top">
                    <label class="notifications" for="smartmockups_notifications"><?php 
_e('Notifications', SMART_MOCKUPS_DOMAIN);
?>
</label>
                </th>
                <td>
                    <input id="smartmockups_notifications" name="smartmockups_notifications" type="checkbox" value="1" <?php 
echo $settings['notifications'] ? 'checked' : '';
?>
 />
                    <label for="smartmockups_notifications"><?php 
_e('Check this to enable scheduled notifications', SMART_MOCKUPS_DOMAIN);
?>
</label>
                </td>
            </tr>
            <tr valign="top">
                <th scope="row" valign="top">
                    <label class="notifications" for="smartmockups_notifications_recurrence"><?php 
_e('Recurrence', SMART_MOCKUPS_DOMAIN);
?>
</label>