time.tzname is initialized from C tzname variable or tm_zone around Jan, Jul of the current year.

If time.mktime() is called with a time tuple from the past/future then after the call time.tzname might be out-of-sync with the corresponding C tzname and tm_zone values. Because C mktime may change tzname, tm_zone values on some systems and time.mktime calls C mktime.

I've attached test_mktime_changes_tzname.c file that demonstrates that mktime() may change tzname.