Yeah, you're correct that `timegm` is neither part of the C99 standard nor officially specified in POSIX.1-2024 but it is widely supported in practice on many platforms, including glibc, musl, and BSD systems which makes it a pragmatic choice in environments where it is available. Additionally, it is easy to implement it in a portable way when unavailable.

So, while `timegm` is not standardized in C99 or POSIX, it is a practical solution in most real-world environments, and alternatives exist for portability, and thus: handling time in C is not inherently a struggle.

As for the link, it says "You may want to bite the bullet and use timegm(3), even though it’s nominally not portable.", but see what I wrote above.