I have seen that all the pokemon with a date before to the 5th gen is marked as legal, however this is impossible, because when transferring the pokemon from 4th to 5th gen by the Poké Transfer, the capture date changes to the day that it was passed by Poké Transfer. It happens the same as when going from 3rd to 4th generation when captured in Pal Park.
Therefore, it is impossible to have in 5th, 6th, 7th and 8th gen a pokemon dated before 09/18/2010, or in 4th gen a pokemon dated before 09/28/2006, but PKHeX marks it as legal.
Moreover, the date never marks it as illegal, for example a scoorbunny with a capture date in 2007 does not mark it as illegal.
