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> C mktime itself should not change timezone globals, but it may indirectly if it calls tzset() and TZ changed between calls.
You should have run the attached test_mktime_changes_tzname.c which demonstrates that (at least on some systems) C mktime *does* change tzname global even if TZ envvar is constant in the test.
Nowhere in my bug report I had mentioned different TZ values. I did mentioned *past* *future* dates -- the same timezone may have different utc offset, timezone abbreviations at different times. These timezone globals can change even if TZ is constant.