These reveal a roughly factor of six slow-down for two valid date
formats and a roughly factor of twelve slow-down for an invalid one.
Pick-to: 6.7 6.5 6.2
Task-number: QTBUG-124465
Change-Id: Ibd21e43d4c64aced33ba5b21e4602e0dc4fd7548
Reviewed-by: Mate Barany <mate.barany@qt.io>