The method of setting the time and date depends on the model of the heating device you have.
If you have new generation heating boiler with WiFi connectivity, it will automatically change to winter or summer time.
For some Viessmann One Base heating devices you can use Settings to have your device adjust automatically for daylight saving time. Verify if this option is selected.
For older ones you can check the summer mode by pressing individual buttons on the HMI of your heating device. This is located directly on the boiler. The display shows the current operating mode.