Again, unsigned ints weren't even present in C until 1978, so of course it was signed -- there was no alternative. As far as I've ever read, negative unix time

Again, unsigned ints weren't even present in C until 1978, so of course it was signed -- there was no alternative. As far as I've ever read, negative unix timestamp values generally were not used. Since -1 was used as an error value, you cannot safely determine whether that value represents an error vs a legitimate timestamp one second before the epoch. I'm not familiar with if/how early versions of Unix were able to interact with non-Unix files/filesystems, but I doubt they would preserve metadata such as mtime if so.