Are Green Day Punk?

As George said to Captain Blackadder, 'I'm as excited as a very excitable person who has a special reason to be excited' and it's all because Green Day have an album coming out in a few days but with each Green Day release, we get the inevitable question 'Are Green Day Punk?'
It's not easy to define Punk music, mostly guitar wailing, fast, hard-edged 3 minute songs that often have angry political or anti-establishment lyrics and Green Day, the early stuff anyway, fit comfortably into that description.
To those who were around in the early days of punk, Green Day are a million miles away from the original punk bands they listened to such as The Damned, The Ramones, Sex Pistols and The Clash, but isn't that the point?
The genre of 1977 should have evolved and spurned new sub-genres of punk almost 40 years later so we go from The Damned to bands like Green Day and Offspring today who in 2012 shouldn't be sounding like a 1970's band and how boring it would be if they did.
Kurt Cobain always considered Nirvana a punk band and Grunge should be considered as one of the styles of music to have come from, and be inspired by, the original punk which brings us back to Green Day.
Obviously their music is inspired by the early punk bands and there is a very clear line from those first bands through the decades to them today so you can call them pop-punk as most seem to, but are they punk?
The biggest argument against them is that they are mainstream and have sold something like 70 million records, have a play on Broadway and are all millionaires so how can they be punks but couldn't that be because they are a decent band singing songs people like to listen to?
You don't sell that many records or become that successful by being bad and singing songs people don't like, if you hit upon a sound that is liked by millions then only a fool would refuse all the rewards that came with that.
So is the Green Day sound punk? Yes, definitely inspired and in the tradition of punk music so to me they are punk, just the 2012 version of it as Nirvana were the 1991 and the Sex Pistols were the 1977 versions.