My first recommendation is "Don't do that".  In order to maintain secure communications, we need to have clocks automatically synchronized.  The MTP panels support using an NTP server to synchronize clocks.  If you don't already have a local NTP server on your network, the LSNTP Library for SNTP Server Functionality in SIMATIC S7 CPUs can set up your PLC CPU as an NTP server.

wincc-unified-simple-sample-working-with-clock-date-time deals with reading the system clock and displaying, but I don't spot where it sets the system clock.  I did not spot any clock setting methods in the system functions or the HMIRuntime objects.  It looks to me like the options for setting the MTP clock are currently limited to via NTP or via the MTP control panel