Just one tip:
Some of Siemens panels only keep the clock for a rather limited time. One example is OP177B. If you shutdown the panel for the weekend, then on monday morning the clock will have reset to the default.
Thus, dont set the clock from the PLC by means of a button.
In stead set it to load with the area pointer every 1 minute.
Only in this way will it automatically retrieve the PLC clock after powerup.