Implementing RFC 3339 shouldn't really be that hard...
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Further reading: RFC 3339 / ISO 8601
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Never not UTC Everywhere.
https://utceverywhere.com/
UTC is better than most, but leap seconds are still awful. Computers should use GPS or TAI everywhere. Dealing with time zones and leap seconds is for human readability and display purposes only.
That's why I only ever use Seconds Since Epoch.