Inside WatchedItem.php,. at the end of resetNotificationTimestamp, there is a call to update a user's watchlist like so:
$dbw->update( 'watchlist', array( 'wl_notificationtimestamp' =>$notificationTimestamp ), $this->dbCond(), METHOD );
As $notificationTimestamp can be a raw string at this point, the call needs to be:
$dbw->update( 'watchlist', array( 'wl_notificationtimestamp' => $dbw->timestamp( $notificationTimestamp ) ), $this->dbCond(), METHOD );