If the TZ is not set, then the auto tests fail like this:

FAIL!  : tst_QTimeZone::systemZone() 'zone.isValid()' returned FALSE. (Invalid system zone setting, tests are doomed on misconfigured system.)
   Loc: [/home/karim/src/vxtimetests/qtimezone/tst_qtimezone.cpp(566)]
but when it is set to TZ=Asia/Kolkata:

FAIL!  : tst_QTimeZone::systemZone() Compared values are not the same
   Actual   (date.startOfDay(QTimeZone::LocalTime)): -4714/11/24 00:00:00.000[GMT+5:30]
   Expected (date.startOfDay(zone))                : -4714/11/24 00:00:00.000[GMT+5:30]
   Loc: [/home/karim/src/vxtimetests/qtimezone/tst_qtimezone.cpp(581)]

Works with TZ=UTC