было приятно узнать, что FreeBSD лишена проблемы by design.

FreeBSD and macOS both support utmpx only, not traditional utmp. But they do support 64-bit timestamps at least on 64-bit architectures, because the ut_tv field of struct utmpx is a struct timeval, which on these systems uses 64-bit or 32-bit timestamps depending on the architecture. FreeBSD has a semi-traditional implementation of getutxent that reads a file, but the on-disk format is a separate struct that their libc manually converts to struct utmpx, so it doesn't matter that struct utmpx is different between 32-bit and 64-bit programs. (This is in contrast to glibc where struct utmp, struct utmpx, and the on-disk format are all identical.)