Canon has recently released a new firmware version for its DSLR EOS 6D camera. This update will fix the date and time reset issue.
Installing the 1.1.4, you will need a fully charged battery, an SD card (with no less than 64 MB capacity) and the firmware file.
Format the memory card with the camera and copy the “fir” file found in the downloaded archive.
Safety remove the device, plug back in your camera, rotate the mode dial to “P” and hit the power switch.
In the menu, under the “Set-up 4 (Yellow)” screen, you will the “firmware version” section. Press the “Set” button to go to the update screen. When asked, approve the installation.
The camera will search the card for available updates and will ask to select the file you want to install. After you've confirmed (once again) that you are willing to update, a progress bar will indicate how things are going.
After the procedure is complete, this will be displayed on the screen. Hit the “Set” button and power off your camera. Remove the battery for at least two seconds. This will cause the firmware to take effect after the battery has been reloaded and the camera is turned on.