We research this concern and here is what we've found out.
Before you can use the accessories on your car, including the clock, you must first turn on the vehicle engine. You can also turn only on the accessory mode of your vehicle to use this.
Newer Hummer H3 models have the CONFIG hard button. If your car happens to have this button, all you need to do is to press it. Other models don't have a CONFIG button, so you need to turn the volume knob to turn on the system.
To adjust the time on newer models with touch screens, you need to tap on the clock screen button. Search for the desired time zone, and it will automatically set the date and time of your car.