I’m surprised by the posters writing that the idea that the first 6 Days of Creation were longer than 24 hours is controversial. I don’t know the source, but I always learned that in school, and I went to very Yeshivish schools when I was younger. I really am under the impression that it’s not a controversial point of view, although I don’t have a source off-hand.
As stated by Sam2, Rav Elyshiv stated anyone who did not belive the world was 6000 years old was a Koifer
I think it was Rav AVigdoe Miller or Rav Shimon Schwab (I could be wrong here) said he belived the world was 6000 years old , he did not think it was Kefirah to belive it was older