Never managed to get on the HIMYM bandwagon so I can't comment on that until I get around to finally watching it.
As far as Seinfeld, I've noticed that people my age who grew up with the show in school and college consider it timeless but a lot of younger people don't see the big deal. Not so much that it's not funny (it is) or that it's dated (cultural references and cordless home phones are just window dressing), but that so many subsequent shows have been influenced by Seinfeld that it doesn't seem very new or original to a 17-year-old who just saw a few reruns for the first time.
It's sort of like how people will talk about The Beatles being overrated because people still refer to their relevance when they were still putting out albums, not how their work relates to current rock music. When the Beatles got huge it was relevant as much for the time and place as it was the actual music. But if you're a young listener today and you hear the Beatles at the same time as you hear later artists who were influenced by the Beatles, they don't seem like a big deal.
That's the problem with "timelessness" and something like pop culture that's always changing and building upon itself (not to mention how it favors novelty). It seems like there will always be cultural "enthusiasts" in any medium who appreciate the influence a given artist has on the popular culture but just the same, I find that same general "popular" audience loses sight of what made something big. And since the nature of pop culture is to mirror and cater to "the masses", on some level, that's who really determines relevance.
I'd agree on the music part. Timeless is something that stays unique, that will never be replicated. The Beatles have been successfully copied and mass-marketed, and are no longer really timeless. "Smells Like Teen Spirit", I would argue, is timeless, in that none of the imitators ever really pinned it down. Kids still hear it and it resonates with them. I would pin Queen as timeless as well, nobody has managed to really get that formula right again, but it still appeals today. Comedy can rarely be 'timeless', because its SO easy to steal a routine or a persona that made a previous comedian beloved, and thus dilute it.
