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

Yes,

• I have checked to ensure there aren't other open Pull Requests for
  the same update/change?
• I have followed the guidelines in our
  Contributing and e.g. bundled a test in the
  function header comments that fails before this PR, but passes after.

[kvz]

Really nicely done, thanks!