The info for "event" Pokémon often claims "it may be obtained on any date, beginning from
when it was released.". Any non-trade Pokémon (except gen 3 things that are pal-parked and I don't think they have done any in gen 4) can legally have any date from January 1st 2000 onward, as their listed date is that of the DS when obtained/pal-parked (1/1/00 being the earliest date the DS can be set to.