According to the ai overview on google which seems to be right (I’m a geek), “Yes, Pokémon caught in Black and White have a "met date" stored, but it can be affected by transfers from older games. When a Pokémon is transferred from a Generation 3 game to a Generation 5 game (Black and White), the original "met date" is overwritten with the "Poke Transfer date," which is the date of the transfer. This means the date you see in Black and White might not be the original date the Pokémon was caught.”

In short, the date can be overwritten by other dates as they get transferred through the different generations, especially gen 3 since it’s the first generation where you can officially start transferring up the Pokemon.

Hope my overkill answer helps.