diff --git a/src/corelib/time/qdatetime.cpp b/src/corelib/time/qdatetime.cpp
index 9690c8c66bf..6bf29bc0d63 100644
--- a/src/corelib/time/qdatetime.cpp
+++ b/src/corelib/time/qdatetime.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2021 The Qt Company Ltd.
+** Copyright (C) 2022 The Qt Company Ltd.
 ** Copyright (C) 2021 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
@@ -4373,13 +4373,13 @@ static inline void massageAdjustedDateTime(QDateTimeData &d, QDate date, QTime t
     if (spec == Qt::LocalTime)
         utc = QDateTimePrivate::localMSecsToEpochMSecs(local, &dst, &date, &time);
 #if QT_CONFIG(timezone)
-    else if (spec == Qt::TimeZone && d->m_timeZone.isValid())
-        utc = QDateTimePrivate::zoneMSecsToEpochMSecs(local, d->m_timeZone, &dst, &date, &time);
+    else if (spec == Qt::TimeZone && d.d->m_timeZone.isValid())
+        utc = QDateTimePrivate::zoneMSecsToEpochMSecs(local, d.d->m_timeZone, &dst, &date, &time);
 #endif // timezone
     else
         dst = QDateTimePrivate::UnknownDaylightTime;
 
-    setDateTime(d, date, time);
+    setDateTime(d, date, time); // Detaches d, if needed.
     status = getStatus(d); // Updated by setDateTime()
     const bool ok = (dst != QDateTimePrivate::UnknownDaylightTime
                      && (status & QDateTimePrivate::ValidDate)
diff --git a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
index 1b12abdcf95..844dead5046 100644
--- a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
@@ -1134,6 +1134,18 @@ void tst_QDateTime::addDays()
     QCOMPARE(dt2.timeSpec(), Qt::OffsetFromUTC);
     QCOMPARE(dt2.offsetFromUtc(), 60 * 60);
 
+#if QT_CONFIG(timezone)
+    const QTimeZone cet("Europe/Oslo");
+    if (cet.isValid()) {
+        dt1 = QDate(2022, 1, 10).startOfDay(cet);
+        dt2 = dt1.addDays(2); // QTBUG-99668: should not assert
+        QCOMPARE(dt2.date(), QDate(2022, 1, 12));
+        QCOMPARE(dt2.time(), QTime(0, 0));
+        QCOMPARE(dt2.timeSpec(), Qt::TimeZone);
+        QCOMPARE(dt2.timeZone(), cet);
+    }
+#endif
+
     // Test last UTC second of 1969 *is* valid (despite being time_t(-1))
     dt1 = QDateTime(QDate(1969, 12, 30), QTime(23, 59, 59), Qt::UTC).toLocalTime().addDays(1);
     QVERIFY(dt1.isValid());