Unfortunately typical but: "The snow in the mountains was melting and Bunny had been dead for several weeks before we came to understand the gravity of our situation”. (The Secret History by Donna Tartt)

"Mother died today. Or maybe it was yesterday, I don't know." (The Stranger by Albert Camus)

And this isn't strictly a first line but is the first line in the beginning of chapter one (there's one page before it that I skipped lol): "All of this happened more or less. The war parts, anyway, are pretty much true." (Slaughterhouse-V by Kurt Vonnegut)