C and D are the Roman numerals corresponding to 100 and 500 respectively. Hence, 5C = D.

But, the larger point was that took 500 (5C) rather than 5000 (5K) years to prove Fermat's Last Theorem.

  I =    1
  V =    5
  X =   10
  L =   50
  C =  100
  D =  500
  M = 1000

  => D = 5C or, equivalently, 5C=D

It's more like 8L years ago, though.