There is no different code there is just one met date in the Pokémon structure . It is overwritten nothing of the original remains. I’ve heard other users speculate there is secret old remnant data left over in transfers, but no that just uses up memory and is useless. Anything not needed during transfer or gets updates that data is just gone. Also you keep mentioning Poke Bank, but Poke Bank does nothing to the met date unless it’s a transfer from a virtual counsel game. Gen 5 is the last time met dates get modified.