Styleguide: Either write time zone safe code or document that it's unsafe

Rendered: https://github.com/henrik/devbook/tree/timezone/styleguide#either-write-time-zone-safe-code-or-document-that-its-unsafe

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- [x] HN
- [x] KP
- [x] JK
- [x] AR
- [x] TS
- [x] VA