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I think that BC is pretty much impossible at the moment and I think it will remain that way. There is a time limit removing patch that increases the timespan from 2-65536, and it does not allow negative numbers, nor can it make the game count in reverse. I expect that AVC was chosen to eliminate the BC/AC-shift.
 
If Paradox wants to make it an option, though, it would be fairly straight forward.

The game always keeps track of everything in AUC, but if you click the 'Display BC' button, then, before it displays the number, it just subtracts 753 from it. That's all.

The engine doesn't have to keep track of negative numbers, nothing internal is changed, it just displays a little differently.