avoid std::get_time usage to sidestep libstdc++6 bug

As reported upstream in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71556
the implementation of std::get_time is currently not as accepting as
strptime is, especially in how hours should be formatted.

Just reverting 9febc2b2 would be
possible, but then we would reopen the problems fixed by it, so instead
I opted here for a rewrite of the parsing logic which makes this method
a lot longer, but at least it provides the same benefits as the rewrite
in std::get_time was intended to give us and decouples us from the fix
of the issue in the standard library implementation of GCC.

LP: 1593583