Fast DateTime class.
The FastDateTime class implements a datetime that is much faster than QDateTime for comparing dates. It caches the linear time since the epoch for use in comparisons. See https://bugreports.qt.io/browse/QTBUG-41714 and https://bugreports.qt.io/browse/QTBUG-75585 for an explanation of why the standard QDateTime is extremely slow (on the order of 1 usec per comparison).
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