fix QDateTime performance on Qt6 #115

Job	Run time
build (ubuntu-latest, Debug, static, 5.12, x86_64)	47s
build (ubuntu-latest, Debug, static, 6.6.0, x86_64;arm64)	1m 24s
build (ubuntu-latest, Debug, shared, 5.12, x86_64)	39s
build (ubuntu-latest, Debug, shared, 6.6.0, x86_64;arm64)	1m 2s
build (ubuntu-latest, Release, static, 5.12, x86_64)	55s
build (ubuntu-latest, Release, static, 6.6.0, x86_64;arm64)	58s
build (ubuntu-latest, Release, shared, 5.12, x86_64)	49s
build (ubuntu-latest, Release, shared, 6.6.0, x86_64;arm64)	53s
build (windows-latest, Debug, static, 5.12, x86_64)	1m 47s
build (windows-latest, Debug, static, 6.6.0, x86_64;arm64)	2m 6s
build (windows-latest, Debug, shared, 5.12, x86_64)	1m 21s
build (windows-latest, Debug, shared, 6.6.0, x86_64;arm64)	1m 34s
build (windows-latest, Release, static, 5.12, x86_64)	1m 40s
build (windows-latest, Release, static, 6.6.0, x86_64;arm64)	1m 32s
build (windows-latest, Release, shared, 5.12, x86_64)	1m 42s
build (windows-latest, Release, shared, 6.6.0, x86_64;arm64)	1m 31s
build (macos-latest, Debug, static, 5.12, x86_64)	31s
build (macos-latest, Debug, static, 6.6.0, x86_64;arm64)	1m 27s
build (macos-latest, Debug, shared, 5.12, x86_64)	40s
build (macos-latest, Debug, shared, 6.6.0, x86_64;arm64)	1m 23s
build (macos-latest, Release, static, 5.12, x86_64)	36s
build (macos-latest, Release, static, 6.6.0, x86_64;arm64)	1m 25s
build (macos-latest, Release, shared, 5.12, x86_64)	33s
build (macos-latest, Release, shared, 6.6.0, x86_64;arm64)	1m 9s
	28m 24s

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