You do need real-time time stamps for some parts of a display protocol, since clients should be able to accurately determine when past frames were displayed, an

You do need real-time time stamps for some parts of a display protocol, since clients should be able to accurately determine when past frames were displayed, and accurately request display times for future frames (subject to the potential for failure because OS scheduling is not hard real-time). This is especially true for modern displays which have some limited support for variable refresh rates.