Setting the time and date depends on the model of your heater:
Viessmann One Base systems
The newer generation of One Base heating systems automatically switch from summer to winter time and vice versa. For some devices, you must first activate the automatic changeover in the settings during commissioning. The individual steps for this can be found in the operating instructions for your specific system.
Older generation heating systems
Depending on the model, you may have to change the time manually. You can check the current operating mode directly on the heating control display on the boiler and adjust it if necessary. Please also use the operating instructions for your appliance for this purpose.