diff --git a/src/corelib/tools/qdatetime.cpp b/src/corelib/tools/qdatetime.cpp
index eeefea8137..e5fbf5af5e 100644
--- a/src/corelib/tools/qdatetime.cpp
+++ b/src/corelib/tools/qdatetime.cpp
@@ -2607,7 +2607,6 @@ QDateTimePrivate::QDateTimePrivate(const QDate &toDate, const QTime &toTime,
 void QDateTimePrivate::setTimeSpec(Qt::TimeSpec spec, int offsetSeconds)
 {
     clearValidDateTime();
-    clearTimeZoneCached();
     clearSetToDaylightStatus();
 
 #ifndef QT_BOOTSTRAPPED
@@ -2707,27 +2706,44 @@ QDateTimePrivate::DaylightStatus QDateTimePrivate::daylightStatus() const
     return UnknownDaylightTime;
 }
 
+qint64 QDateTimePrivate::toMSecsSinceEpoch() const
+{
+    switch (m_spec) {
+    case Qt::OffsetFromUTC:
+    case Qt::UTC:
+        return (m_msecs - (m_offsetFromUtc * 1000));
+
+    case Qt::LocalTime:
+        // recalculate the local timezone
+        return localMSecsToEpochMSecs(m_msecs);
+
+    case Qt::TimeZone:
+#ifndef QT_BOOTSTRAPPED
+        return zoneMSecsToEpochMSecs(m_msecs, m_timeZone);
+#endif
+        break;
+    }
+    Q_UNREACHABLE();
+    return 0;
+}
+
 // Check the UTC / offsetFromUTC validity
 void QDateTimePrivate::checkValidDateTime()
 {
     switch (m_spec) {
     case Qt::OffsetFromUTC:
     case Qt::UTC:
+        // for these, a valid date and a valid time imply a valid QDateTime
         if (isValidDate() && isValidTime())
             setValidDateTime();
         else
             clearValidDateTime();
         break;
     case Qt::TimeZone:
-        // Defer checking until required as can be expensive
-        clearValidDateTime();
-        clearTimeZoneCached();
-        m_offsetFromUtc = 0;
-        break;
     case Qt::LocalTime:
-        // Defer checking until required as can be expensive
-        clearValidDateTime();
-        m_offsetFromUtc = 0;
+        // for these, we need to check whether the timezone is valid and whether
+        // the time is valid in that timezone. Expensive, but no other option.
+        refreshDateTime();
         break;
     }
 }
@@ -2741,12 +2757,6 @@ void QDateTimePrivate::refreshDateTime()
         // Always set by setDateTime so just return
         return;
     case Qt::TimeZone:
-        // If already cached then don't need to refresh as tz won't change
-        if (isTimeZoneCached())
-            return;
-        // Flag that will have a cached result after calculations
-        setTimeZoneCached();
-        break;
     case Qt::LocalTime:
         break;
     }
@@ -3082,7 +3092,6 @@ bool QDateTime::isNull() const
 
 bool QDateTime::isValid() const
 {
-    d->refreshDateTime();
     return (d->isValidDateTime());
 }
 
@@ -3143,13 +3152,11 @@ Qt::TimeSpec QDateTime::timeSpec() const
 QTimeZone QDateTime::timeZone() const
 {
     switch (d->m_spec) {
-    case Qt::OffsetFromUTC:
-        if (!d->m_timeZone.isValid())
-            d->m_timeZone = QTimeZone(d->m_offsetFromUtc);
-        return d->m_timeZone;
     case Qt::UTC:
         return QTimeZone::utc();
+    case Qt::OffsetFromUTC:
     case Qt::TimeZone:
+        Q_ASSERT(d->m_timeZone.isValid());
         return d->m_timeZone;
     case Qt::LocalTime:
         return QTimeZone::systemTimeZone();
@@ -3178,7 +3185,6 @@ QTimeZone QDateTime::timeZone() const
 
 int QDateTime::offsetFromUtc() const
 {
-    d->refreshDateTime();
     return d->m_offsetFromUtc;
 }
 
@@ -3350,7 +3356,7 @@ void QDateTime::setTimeZone(const QTimeZone &toZone)
     d->m_spec = Qt::TimeZone;
     d->m_offsetFromUtc = 0;
     d->m_timeZone = toZone;
-    d->m_status = d->m_status & ~QDateTimePrivate::ValidDateTime & ~QDateTimePrivate::TimeZoneCached;
+    d->refreshDateTime();
 }
 #endif // QT_BOOTSTRAPPED
 
@@ -3371,7 +3377,6 @@ void QDateTime::setTimeZone(const QTimeZone &toZone)
 */
 qint64 QDateTime::toMSecsSinceEpoch() const
 {
-    d->refreshDateTime();
     return d->toMSecsSinceEpoch();
 }
 
@@ -3453,8 +3458,8 @@ void QDateTime::setMSecsSinceEpoch(qint64 msecs)
         d->m_status = d->m_status
                     | QDateTimePrivate::ValidDate
                     | QDateTimePrivate::ValidTime
-                    | QDateTimePrivate::ValidDateTime
-                    | QDateTimePrivate::TimeZoneCached;
+                    | QDateTimePrivate::ValidDateTime;
+        d->refreshDateTime();
 #endif // QT_BOOTSTRAPPED
         break;
     case Qt::LocalTime: {
diff --git a/src/corelib/tools/qdatetime.h b/src/corelib/tools/qdatetime.h
index 784aced71a..88288872df 100644
--- a/src/corelib/tools/qdatetime.h
+++ b/src/corelib/tools/qdatetime.h
@@ -326,7 +326,7 @@ private:
     // ### Qt6: Using a private here has high impact on runtime
     // on users such as QFileInfo. In Qt 6, the data members
     // should be inlined.
-    QExplicitlySharedDataPointer<QDateTimePrivate> d;
+    QSharedDataPointer<QDateTimePrivate> d;
 
 #ifndef QT_NO_DATASTREAM
     friend Q_CORE_EXPORT QDataStream &operator<<(QDataStream &, const QDateTime &);
diff --git a/src/corelib/tools/qdatetime_p.h b/src/corelib/tools/qdatetime_p.h
index a139390a9d..b8934f7f70 100644
--- a/src/corelib/tools/qdatetime_p.h
+++ b/src/corelib/tools/qdatetime_p.h
@@ -79,10 +79,9 @@ public:
     enum StatusFlag {
         NullDate            = 0x01,
         NullTime            = 0x02,
-        ValidDate           = 0x04,
-        ValidTime           = 0x08,
-        ValidDateTime       = 0x10,
-        TimeZoneCached      = 0x20,
+        ValidDate           = 0x04, // just the date field
+        ValidTime           = 0x08, // just the time field
+        ValidDateTime       = 0x10, // the whole object (including timezone)
         SetToStandardTime   = 0x40,
         SetToDaylightTime   = 0x80
     };
@@ -120,7 +119,7 @@ public:
     DaylightStatus daylightStatus() const;
 
     // Returns msecs since epoch, assumes offset value is current
-    inline qint64 toMSecsSinceEpoch() const { return (m_msecs - (m_offsetFromUtc * 1000)); }
+    inline qint64 toMSecsSinceEpoch() const;
 
     void checkValidDateTime();
     void refreshDateTime();
@@ -133,14 +132,11 @@ public:
     inline bool isValidDateTime() const { return m_status & ValidDateTime; }
     inline void setValidDateTime() { m_status |= ValidDateTime; }
     inline void clearValidDateTime() { m_status &= ~ValidDateTime; }
-    inline bool isTimeZoneCached() const { return m_status & TimeZoneCached; }
-    inline void setTimeZoneCached() { m_status |= TimeZoneCached; }
-    inline void clearTimeZoneCached() { m_status &= ~TimeZoneCached; }
     inline void clearSetToDaylightStatus() { m_status &= ~(SetToStandardTime | SetToDaylightTime); }
 
 #ifndef QT_BOOTSTRAPPED
     static qint64 zoneMSecsToEpochMSecs(qint64 msecs, const QTimeZone &zone,
-                                        QDate *localDate, QTime *localTime);
+                                        QDate *localDate = 0, QTime *localTime = 0);
 #endif // QT_BOOTSTRAPPED
 
     static inline qint64 minJd() { return QDate::minJd(); }
diff --git a/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
index c6806086d3..446e56e936 100644
--- a/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
@@ -142,6 +142,9 @@ private slots:
     void isDaylightTime() const;
     void daylightTransitions() const;
     void timeZones() const;
+#if defined(Q_OS_UNIX)
+    void systemTimeZoneChange() const;
+#endif
 
     void invalid() const;
 
@@ -2984,6 +2987,56 @@ void tst_QDateTime::timeZones() const
     QCOMPARE(future.offsetFromUtc(), 28800);
 }
 
+#if defined(Q_OS_UNIX)
+// Currently disabled on Windows as adjusting the timezone
+// requires additional privileges that aren't normally
+// enabled for a process. This can be achieved by calling
+// AdjustTokenPrivileges() and then SetTimeZoneInformation(),
+// which will require linking to a different library to access that API.
+static void setTimeZone(const QByteArray &tz)
+{
+    qputenv("TZ", tz);
+    ::tzset();
+
+// following left for future reference, see comment above
+// #if defined(Q_OS_WIN32)
+//     ::_tzset();
+// #endif
+}
+
+void tst_QDateTime::systemTimeZoneChange() const
+{
+    struct ResetTZ {
+        QByteArray original;
+        ResetTZ() : original(qgetenv("TZ")) {}
+        ~ResetTZ() { setTimeZone(original); }
+    } scopedReset;
+
+    // Set the timezone to Brisbane time
+    setTimeZone(QByteArray("AEST-10:00"));
+
+    QDateTime localDate = QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::LocalTime);
+    QDateTime utcDate = QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::UTC);
+    QDateTime tzDate = QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), QTimeZone("Australia/Brisbane"));
+    qint64 localMsecs = localDate.toMSecsSinceEpoch();
+    qint64 utcMsecs = utcDate.toMSecsSinceEpoch();
+    qint64 tzMsecs = tzDate.toMSecsSinceEpoch();
+
+    // check that Australia/Brisbane is known
+    QVERIFY(tzDate.timeZone().isValid());
+
+    // Change to Indian time
+    setTimeZone(QByteArray("IST-05:30"));
+
+    QCOMPARE(localDate, QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::LocalTime));
+    QVERIFY(localMsecs != localDate.toMSecsSinceEpoch());
+    QCOMPARE(utcDate, QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::UTC));
+    QCOMPARE(utcDate.toMSecsSinceEpoch(), utcMsecs);
+    QCOMPARE(tzDate, QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), QTimeZone("Australia/Brisbane")));
+    QCOMPARE(tzDate.toMSecsSinceEpoch(), tzMsecs);
+}
+#endif
+
 void tst_QDateTime::invalid() const
 {
     QDateTime invalidDate = QDateTime(QDate(0, 0, 0), QTime(-1, -1, -1));