Bug discovered by @xTibor. Test and input data provided by him.

Apparently gmtime was already implemented when I made localtime, so we had two different things written from scratch. We decided in the relibc channel of the Redox OS Mattermost chat to use my code, as it is more extensively tested and perhaps is clearer in how it works.

I really really wish I could actually test this on redox. All I know is: it compiles