The clock / time setting changes on its own on my car stereo

Check the Setting Clock Time (CT) setting of the menu.

If the Setting Clock Time (CT) function is ON, the time is set to the data received from the radio station, regardless of the correctness.

The following steps are an example of how to  set the clock.
For details, refer to the manual supplied with the product.

1. Press HOME, then touch Settings.
2. Touch   then touch Clock Adjust. The setting display appears.
3. Touch  to set the hour and minute.
4. After setting the hour and minute, touch Set.
5. The setting is complete and the clock starts.