I am also puzzled by this. I thought with early access extended to the 6th my 30 days wouldnt start til then either, but it says i will be billed on the 4th as well.

I emailed support and got this response

"Greetings!

Thank you for contacting The Elder Scrolls Online Team.

The early access has ended, people who bought from a store were given early access until the 6th to enter in their code."

They didn't really answer my question as to whether that meant that people who waited would be getting 2 more days of playtime, or whether that was just "grace period" time that would be subtracted from the 30 days once they entered their key.

2 days isn't a huge deal or anything, but will be kind of annoying if it turns out that registering the key promptly cost us time.