Having a time travel problem with your Canon 6D? A recent firmware update should bring you up to date, as well as prevent it from happening in the future:
Firmware Version 1.1.3 Incorporates the Following Fix 1. Fixes a phenomenon in which the Date/Time/Zone settings screen appears on the LCD display, after the user has already configured these settings. The values for the Date/Time settings may reset if the backup functions which retain those values do not perform properly. Even if the camera does not exhibit this phenomenon currently, it may in the future. Accordingly, we recommend that you update the Firmware in the camera to version 1.1.3 as soon as possible. Firmware Version 1.1.3 is for cameras with firmware up to version 1.1.2. If the camera’s firmware is already Version 1.1.3, it is not necessary to update the firmware. Before downloading the firmware update for your camera, please review the instructions thoroughly. -via Canon Rumors