The UTC time-zone never changes, not even under a locale change, so memoize utc() using a Q_GLOBAL_STATIC cache. Check its isDestroyed() and fall back to the current implementation in that case. This keeps the function available even in late shutdown stages. _This_ is a knife^Woptimization: PASS : tst_QTimeZone::utc() RESULT : tst_QTimeZone::utc(): - 33.1472430 nsecs per iteration (total: 331,472,431, iterations: 10000000) + 12.8953219 nsecs per iteration (total: 128,953,219, iterations: 10000000) - 150.5929452 CPU cycles per iteration, 4,54 GHz (total: 1,505,929,452, iterations: 10000000) + 60.3382987 CPU cycles per iteration, 4,68 GHz (total: 603,382,988, iterations: 10000000) - 396.0402927 instructions per iteration, 2,630 instr/cycle (total: 3,960,402,928, iterations: 10000000) + 84.0138436 instructions per iteration, 1,392 instr/cycle (total: 840,138,437, iterations: 10000000) - 96.0069811 branch instructions per iteration, 2,9 G/sec (total: 960,069,811, iterations: 10000000) + 28.0024131 branch instructions per iteration, 2,17 G/sec (total: 280,024,132, iterations: 10000000) Amends HEAD^. Change-Id: I4496f3a4c1f64b0615930337662bcd04af0a2ade Reviewed-by: Thiago Macieira <thiago.macieira@intel.com> Reviewed-by: Milian Wolff <milian.wolff@kdab.com> Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>