Working as intended; receiving a .wcxfull naturally sets the gift date to the current date of the system.
You can always have a desync'd 3DS clock, so any date-legality checking you may have been advised on is incorrect.
If you can't be bothered with changing your system clock, just import the gift file to the main window and PKHeX will create a Pokémon the same way the game does, after which you can set the date to whatever you want.