You can make it an entry field just put it on a high level password for engineering, so only those who know the password can change the date & time, you will probably find that the clock is not that accurate anyway, I have seen HMI's where the clock gains/looses minutes a week so it makes sense to be able to correct it from time to time, some HMI's can sync from a server but I don't think you are connecting it to your network.
Not used the later Siemens HMI's but some also have an engineering screen you can access during runtime again make it high level acces so bubber can't get to it.