When QDateTime::addDate() and friends sanitize their end-state, they
were using the DST status of their start-state (if known) to control
it. This lead to misguided results and, in particular, inconsistent
results given that a raw-constructed QDateTime comes into being
ignorant of its DST, while a .toLocalTime() one knows its DST.
Furthermore, the code to do this was triplicated, tricky and poorly
explained. So pull it out into a local static function and explain
what it's doing, and why, more clearly and only once.
Task-number: QTBUG-49008
Change-Id: Ia4bb3c5e9267fff8bb963ea705267998218ed623
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>
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