Fix strftime implementations for format strings "%F" and "%j"

[markusmarchewa]

Description

1.) "%F" is supposed to be same as "%Y-%m-%d" (https://www.php.net/manual/en/function.strftime.php) but was "%y-…"

2.) "%j" (day of the year) was wrong in locales with DST shifts because the interval between the start of a day during DST and the start of the year might be less than a multiple of 24 hours (the day when DST starts has just 23 hours and the day when it ends has 25 hours). Compensate by talking the timezone offsets into calculation. The test changes process.env.TZ temporarily.

Checklist

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  the same update/change?
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  function header comments that fails before this PR, but passes after.