Canon appears to have stopped using user-replaceable date/time backup batteries in EOS bodies around 2012. Models such as the 5D Mark III and original 1D X still had one, while later cameras like the 6D, 7D Mark II, 5D Mark IV, and likely the 1D X Mark II do not.

The manuals’ wording suggests newer cameras use an internal backup method such as a capacitor or built-in rechargeable backup cell rather than a removable coin battery. Canon also notes that date/time/zone settings may reset if the camera is stored without the battery, if the battery is exhausted, or after prolonged exposure to very cold temperatures.

So, for the 1D X Mark II, it’s best to assume there is no user-serviceable backup battery. The backup power should be enough for battery changes and possibly short-term storage, but not indefinite storage without a main battery. Exact retention time is not clearly documented; community experience on similar Canon bodies suggests it can last at least days and sometimes a couple of weeks, but you should expect to reset the clock after longer periods.