(it is)

The past.  Unreal.  This is literally true.  Really.

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If one thinks about it, the operation of subtracting a larger number from a smaller one is problematic.  It forces us to run around the bases, and homininis are not wired well for number when fresh out of the packaging.  If you want to test this on yourself, try giving yourself three seconds to memorize a sequence of random numbers with N digits: one would be impressed if you were to manage N > 14.  Try it!

Prehistory is likewise problematic.