Canon EOS-1D X Mark III Advanced User's Manual page 289

Start Time Setting
You can set the initial time of the time code.
Manual input setting
Enables you to set any starting hour, minute, second, and frame.
Reset
Resets the time set with [Manual input setting] or [Set to camera time] to "00:00:00."
or "00:00:00:" ( ).
Set to camera time
Matches the hour, minute, and second set as the time on the camera. Sets the frame
value to "00."
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