Canon EOS R5C Manual page 646

Caution
The date, time, and time zone settings may be reset when the camera is stored
without the battery, when the battery is exhausted, or when it is exposed to freezing
temperatures for an extended period. If this happens, set them once again.
After changing [Time zone], check that the correct date/time are set.
Note
Auto power off time may be extended while the [ : Date/Time/Zone] screen is
displayed.
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