V4: Handle QTime->DateObject conversion better

By specification, date conversion functions for dates before the epoch
are not DST corrected. We converted QTime to a QDateTime where we set
the date part to Jan. 1, 1970, and then convert that to msecs since the
epoch UTC. For places on Earth where they had DST on that day (e.g.
Hobart in Australia), strange things happen: conversion from a QTime to
DateObject will use DST (because it's after the epoch in local time),
but conversions from DateObject to QTime won't use the DST because it's
before the epoch (in UTC).

Now as everyone knows, a 24-hour clock time has no meaning without a
date, only "elapsed time" has. But users still expect to be able to pass
QTime to QML/JS. So, we do the conversion on day 0 of month 0 of year 0,
and all of it in local time. This gives a stable conversion in both
directions, and the values in both C++ and QML/JS are the same for any
timezone (with or without DST) on this planet.

Task-number: QTBUG-54378
Change-Id: I892e16a93f015e92d311c6cae3ae7768b7373f6a
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

2 files changed, 118 insertions, 2 deletions

diff --git a/tests/auto/qml/qqmlqt/data/timeRoundtrip.qml b/tests/auto/qml/qqmlqt/data/timeRoundtrip.qml
new file mode 100644
index 0000000000..9d73640c87
--- /dev/null
+++ b/tests/auto/qml/qqmlqt/data/timeRoundtrip.qml
@@ -0,0 +1,8 @@
+import QtQuick 2.0
+
+QtObject {
+    Component.onCompleted: {
+        var t = tp.time;
+        tp.time = t;
+    }
+}
diff --git a/tests/auto/qml/qqmlqt/tst_qqmlqt.cpp b/tests/auto/qml/qqmlqt/tst_qqmlqt.cpp
index 69791085c5..0576650d01 100644
--- a/tests/auto/qml/qqmlqt/tst_qqmlqt.cpp
+++ b/tests/auto/qml/qqmlqt/tst_qqmlqt.cpp
@@ -46,6 +46,15 @@
 #include <QFont>
 #include "../../shared/util.h"
 
+// Copied from tst_qdatetime.cpp
+#ifdef Q_OS_WIN
+# include <qt_windows.h>
+# include <time.h>
+#  if defined(Q_OS_WINRT)
+#    define tzset()
+#  endif
+#endif
+
 class tst_qqmlqt : public QQmlDataTest
 {
     Q_OBJECT
@@ -93,6 +102,9 @@ private slots:
     void later();
     void qtObjectContents();
 
+    void timeRoundtrip_data();
+    void timeRoundtrip();
+
 private:
     QQmlEngine engine;
 };
@@ -873,8 +885,6 @@ void tst_qqmlqt::dateTimeFormattingVariants_data()
 
     QTime time(11, 16, 39, 755);
     temporary = QDateTime(QDate(1970,1,1), time);
-    QTest::newRow("formatDate, qtime") << "formatDate" << QVariant::fromValue(time) << (QStringList() << temporary.date().toString(Qt::DefaultLocaleShortDate) << temporary.date().toString(Qt::DefaultLocaleLongDate) << temporary.date().toString("ddd MMMM d yy"));
-    QTest::newRow("formatDateTime, qtime") << "formatDateTime" << QVariant::fromValue(time) << (QStringList() << temporary.toString(Qt::DefaultLocaleShortDate) << temporary.toString(Qt::DefaultLocaleLongDate) << temporary.toString("M/d/yy H:m:s a"));
     QTest::newRow("formatTime, qtime") << "formatTime" << QVariant::fromValue(time) << (QStringList() << temporary.time().toString(Qt::DefaultLocaleShortDate) << temporary.time().toString(Qt::DefaultLocaleLongDate) << temporary.time().toString("H:m:s a") << temporary.time().toString("hh:mm:ss.zzz"));
 
     QDate date(2011,5,31);
@@ -1154,6 +1164,104 @@ void tst_qqmlqt::qtObjectContents()
     delete object;
 }
 
+class TimeProvider: public QObject
+{
+    Q_OBJECT
+    Q_PROPERTY(QTime time READ time WRITE setTime NOTIFY timeChanged)
+
+public:
+    TimeProvider(const QTime &t)
+        : m_getTime(t)
+    {}
+
+    QTime time() const { return m_getTime; }
+    void setTime(const QTime &t) { m_putTime = t; emit timeChanged(); }
+
+signals:
+    void timeChanged();
+
+public:
+    QTime m_getTime, m_putTime;
+};
+
+class TimeZoneSwitch
+{
+public:
+    TimeZoneSwitch(const char *newZone)
+        : doChangeZone(qstrcmp(newZone, "localtime") == 0)
+    {
+        if (!doChangeZone)
+            return;
+
+        hadOldZone = qEnvironmentVariableIsSet("TZ");
+        if (hadOldZone) {
+            oldZone = qgetenv("TZ");
+        }
+        qputenv("TZ", newZone);
+        tzset();
+    }
+
+    ~TimeZoneSwitch()
+    {
+        if (!doChangeZone)
+            return;
+
+        if (hadOldZone)
+            qputenv("TZ", oldZone);
+        else
+            qunsetenv("TZ");
+        tzset();
+    }
+
+private:
+    bool doChangeZone;
+    bool hadOldZone;
+    QByteArray oldZone;
+};
+
+void tst_qqmlqt::timeRoundtrip_data()
+{
+    QTest::addColumn<QTime>("time");
+
+    // Local timezone:
+    QTest::newRow("localtime") << QTime(0, 0, 0);
+
+    // No DST:
+    QTest::newRow("UTC") << QTime(0, 0, 0);
+    QTest::newRow("Europe/Amsterdam") << QTime(1, 0, 0);
+    QTest::newRow("Asia/Jakarta") << QTime(7, 0, 0);
+
+    // DST:
+    QTest::newRow("Namibia/Windhoek") << QTime(1, 0, 0);
+    QTest::newRow("Australia/Adelaide") << QTime(10, 0, 0);
+    QTest::newRow("Australia/Hobart") << QTime(10, 0, 0);
+    QTest::newRow("Pacific/Auckland") << QTime(12, 0, 0);
+    QTest::newRow("Pacific/Samoa") << QTime(13, 0, 0);
+}
+
+void tst_qqmlqt::timeRoundtrip()
+{
+#ifdef Q_OS_WIN
+    QSKIP("On Windows, the DateObject doesn't handle DST transitions correctly when the timezone is not localtime."); // I.e.: for this test.
+#endif
+
+    TimeZoneSwitch tzs(QTest::currentDataTag());
+    QFETCH(QTime, time);
+
+    TimeProvider tp(time);
+
+    QQmlEngine eng;
+    eng.rootContext()->setContextProperty(QLatin1String("tp"), &tp);
+    QQmlComponent component(&eng, testFileUrl("timeRoundtrip.qml"));
+    QObject *obj = component.create();
+    QVERIFY(obj != 0);
+
+    // QML reads m_getTime and saves the result as m_putTime; this should come out the same, without
+    // any perturbation (e.g. by DST effects) from converting from QTime to V4's Date and back
+    // again.
+    QCOMPARE(tp.m_getTime, tp.m_putTime);
+}
+
 QTEST_MAIN(tst_qqmlqt)
 
 #include "tst_qqmlqt.moc"