Convert QTime to V4 Date on a date with no DST in force

Sadly the third centennary of Cassini's birth isn't such a good date
after all, since round-tripping will ignore DST on the QDateTime leg
but include it in V4's implementation of ES's ill-conceived spec for
handling DST and time-zone transitions; and June 8th is in summer for
much of the world. So use a date which (as far as I can tell) appears
to have been DST-free everywhere; and after the start of 1970 so that,
even if we find ourselves working with a zone in which it did have
DST, we'll apply DST consistently during round-tripping.

Fixes: QTBUG-78706
Change-Id: I25be2548e2c97eae2c4f6c5bbe661dea2f35aaba
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 9 insertions, 7 deletions

diff --git a/src/qml/jsruntime/qv4dateobject.cpp b/src/qml/jsruntime/qv4dateobject.cpp
index bba88e5c9a..5616b1450b 100644
--- a/src/qml/jsruntime/qv4dateobject.cpp
+++ b/src/qml/jsruntime/qv4dateobject.cpp
@@ -729,14 +729,16 @@ void Heap::DateObject::init(const QTime &time)
      * time from it, which shall (via toQDateTime(), below) discard the date
      * part.  We need a date for which time-zone data is likely to be sane (so
      * MakeDay(0, 0, 0) was a bad choice; 2 BC, December 31st is before
-     * time-zones were standardized), with no transition nearby in date.  We
-     * ignore DST transitions before 1970, but even then zone transitions did
-     * happen.  Some do happen at new year, others on DST transitions in spring
-     * and autumn; so pick the three hundredth anniversary of the birth of
-     * Giovanni Domenico Cassini (1625-06-08), whose work first let us
-     * synchronize clocks tolerably accurately at distant locations.
+     * time-zones were standardized), with no transition nearby in date.
+     * QDateTime ignores DST transitions before 1970, but even then zone
+     * transitions did happen; and DaylightSavingTA() will include DST, at odds
+     * with QDateTime.  So pick a date since 1970 and prefer one when no zone
+     * was in DST.  One such interval (according to the Olson database, at
+     * least) was 1971 March 15th to April 17th.  Since converting a time to a
+     * date-time without specifying a date is foolish, let's use April Fools'
+     * day.
      */
-    static const double d = MakeDay(1925, 5, 8);
+    static const double d = MakeDay(1971, 3, 1);
     double t = MakeTime(time.hour(), time.minute(), time.second(), time.msec());
     date = TimeClip(UTC(MakeDate(d, t), internalClass->engine->localTZA));
 }