Just because Green Day used punk elements doesn't mean they were ever a punk band. Don't get me wrong, Green Day is a VERY good band. But they are a pop band. If you want to call Green Day punk we may as well throw in Blink 182 as well.
It's a point of contention that may just belie my own pretension, but I've never felt that a band that runs straight to MTV with their first album, or affects a british accent to garner cred, or is focused on the "fashion" of punk, could ever be a punk band. They were one of the first in a frustrating wave of pop-punk that kind of signalled the death-through-success of the 80's punk scene.
Like I said though, very good pop band. But not a punk band.