timegm was finally standardized by C23, and POSIX-2024 mentions it in the FUTURE DIRECTIONS section of mktime. I don't know precisely what happened with POSIX. I think timegm got lost in the shuffle and by the time Austin Group attention turned back to it, it made more sense to let C23 pick it up first so there were no accidental conflicts in specification.[1]
[1] POSIX-2024 incorporates C17, not C23, but in practice the typical POSIX environment going forward will likely be targeting POSIX-2024 + C23, or just POSIX-2024 + extensions; and hopefully neither POSIX nor C will wait as long between standard updates as previously.
[1] POSIX-2024 incorporates C17, not C23, but in practice the typical POSIX environment going forward will likely be targeting POSIX-2024 + C23, or just POSIX-2024 + extensions; and hopefully neither POSIX nor C will wait as long between standard updates as previously.