The system time should be synchronized with the time of the KNX precision clock.
You can also write to the system data point “Time” or “TimeUTC” (provided that the respective W flag at the address is set). So just connect the NominalValue of the system data point to the ActualValue of the KNX data point and you’re done!
Note: Setting the system time requires special rights (in the local security settings you can specify the accounts that are allowed to do this); therefore, this will only work under Win7/8 as long as the Elvis server is running as a service.
It should also be noted that the Elvis server accepts PC time deviations of min. -5 min and max. approx. +10 min, i.e. if you try to advance the PC clock by more than five minutes or by more than ten minutes, the server gets out.