Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 695

Syncing Time Between Cameras Preparing for Time Syncing Syncing the Time You can set the sender camera time on up to 10 receiver cameras. Note that even after synchronization, a slight margin of error applies between sender and receiver camera time (of ±0.05 seconds, at most). Caution Always use cameras of the same model to sync the time. Synchronizing the time on receiver cameras is not possible with different models of the sender and receiver cameras. This feature will not be available after 2038. 695