fix: ts_not_null now works for all timezones

[Jaskowicz1]

This PR fixes the ts_not_null unittest breaking on a non-UTC machine. It also fixes set_ts_not_null.

For more understanding, mktime returns a local time, not a UTC time. It's also not based off your BIOS time, as previously assumed in #1063, but rather the user-level time.

Fixes #1063

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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