replace older time functions with more portable ones

[Gargaj]

As per https://www.gnu.org/software/libc/manual/html_node/Broken_002ddown-Time.html

[epezent]

@Gargaj I think your code is missing an important step:

Portability note: mktime is essentially universally available. timegm is rather rare. For the most portable conversion from a UTC broken-down time to a simple time, set the TZ environment variable to UTC, call mktime, then set TZ back.

[Gargaj]

This has been a while so I don't specifically remember what was being broken, but the functions I was trying to fix were not available on one of the current generation consoles.