Punk rock begins with:
Iggy & The Stooges
Suicide
Richard Hell & the Voidoids
Sex Pistols
Etc...
I wouldn't even include The Clash personally(but that's just opinion).
As for greenday and such, man they just dress sortof like those guys did, but their music is much closer to what you'd have to call "powerpop"
The great thing about recorded music is that it's there long after the artists who made it are dead or burnt out, and punk still means what it did then, 'cos you can still buy those records today.
I got nothing against all these newer bands, but you should realize that they're cashing in on an image that was real important for a while but died out by the early 80's - nevertheless the term "punk" was first applied to a style of music as early as the very late 60's.
Blink 182 etc are just making pop music, with as you say half shouted vocals and wearing some safety pins here and there, because to the kids of today that seems really different to what was going on before, because, as you say they weren't alive then. This has made them (the bands doing it today) real successful($$$$).
Still, if you wanna hear raw punk rock at it's finest, get "Raw power" by Iggy and the Stooges, or the single "Blank Generation" by Richard Hell and the Voidoids, and of course, NEVER MIND THE BOLLOCKS by the sex pistols!! (oh do play that one LOUD!!) Compare any of those tunes with anything that blink 182 or greenday ever did, and then see if they're even slightly similar
I highly recommend:
"Frankie teardrop", Suicide(those guys didn't even have guitar)
"No Fun", Iggy & the Stooges.
And from later on:
"Pretty Vacant", The Sex Pistols
Even if you just hear those few tracks you'll get a damn good idea of what the original punk rock was like, and see why no-one who's heard them could really think of blink 182 or greenday, sum41 or really anything that's going on today as punk rock
Punk rock didn't advocate anything, it just didn't care enough to, singing about opression and such was the province of John lennon and that gang, who were hated by the punk rockers on principle (though some of them later admitted to loving the beatles) This is the reason why I wouldn't class "the Clash" as Punk
So there is a pretty reasonable description of what punk rock was/is, but
In the end, music is music, and so long as you enjoy it what does it matter what it's called?
ZX Spectrum 48k Issue 3, Radio shack Tape drive, Rank arena 12" T.V. set.