The linked post by RNE reports that the AU account shows the same opening date as the primary card.
Base on the below post, AU cards will have their own opening date, since C1 gives AU cards different #s than primary card (at least for me).
https://www.flyertalk.com/forum/35393846-post5.html
Unfortunately I added my AU cards on the same date that I opened the card, so I can't confirm or deny 😌.