diff --git a/src/corelib/time/qdatetime.cpp b/src/corelib/time/qdatetime.cpp
index 53e09873025..ec3c37e740f 100644
--- a/src/corelib/time/qdatetime.cpp
+++ b/src/corelib/time/qdatetime.cpp
@@ -3533,7 +3533,7 @@ QDateTime::Data QDateTimePrivate::create(QDate toDate, QTime toTime, const QTime
     possible. On Windows, where the system doesn't support historical timezone
     data, historical accuracy is not maintained with respect to timezone
     transitions, notably including DST. However, building Qt with the ICU
-    library will equipe QTimeZone with the same timezone database as is used on
+    library will equip QTimeZone with the same timezone database as is used on
     Unix.
 
     \section2 Timezone transitions
@@ -3598,6 +3598,25 @@ QDateTime::Data QDateTimePrivate::create(QDate toDate, QTime toTime, const QTime
     inaccurate. Furthermore, for future dates, the local time zone's offsets and
     DST rules may change before that date comes around.
 
+    \section3 Whole day transitions
+
+    A small number of zones have skipped or repeated entire days as part of
+    moving The International Date Line across themselves. For these, daysTo()
+    will be unaware of the duplication or gap, simply using the difference in
+    calendar date; in contrast, msecsTo() and secsTo() know the true time
+    interval. Likewise, addMSecs() and addSecs() correspond directly to elapsed
+    time, where addDays(), addMonths() and addYears() follow the nominal
+    calendar, aside from where landing in a gap or duplication requires
+    resolving an ambiguity or invalidity due to a duplication or omission.
+
+    \note Days "lost" during a change of calendar, such as from Julian to
+    Gregorian, do not affect QDateTime. Although the two calendars describe
+    dates differently, the successive days across the change are described by
+    consecutive QDate instances, each one day later than the previous, as
+    described by either calendar or by their toJulianDay() values. In contrast,
+    a zone skipping or duplicating a day is changing its description of \e time,
+    not date, for all that it does so by a whole 24 hours.
+
     \section2 Offsets From UTC
 
     Offsets from UTC are measured in seconds east of Greenwich. The moment
diff --git a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
index 17473d4af92..c03c7a89e9d 100644
--- a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
@@ -10,7 +10,14 @@
 #include <private/qtenvironmentvariables_p.h> // for qTzSet(), qTzName()
 
 #ifdef Q_OS_WIN
-#   include <qt_windows.h>
+#  include <qt_windows.h>
+#  if !QT_CONFIG(icu)
+// The native MS back-end for time-zones lacks info about historic transitions:
+#    define INADEQUATE_TZ_DATA
+#  endif
+#endif
+#ifdef Q_OS_ANDROID // Also seems to lack full-day zone transitions:
+#  define INADEQUATE_TZ_DATA
 #endif
 
 using namespace Qt::StringLiterals;
@@ -1289,7 +1296,50 @@ void tst_QDateTime::addDays()
         QCOMPARE(dt2.timeSpec(), Qt::TimeZone);
         QCOMPARE(dt2.timeZone(), cet);
     }
-#endif
+#  ifndef INADEQUATE_TZ_DATA
+    if (const QTimeZone lint("Pacific/Kiritimati"); lint.isValid()) {
+        // Line Islands Time skipped Sat 1994-12-31:
+        dt1 = QDateTime(QDate(1994, 12, 30), QTime(12, 0), lint);
+        dt2 = QDateTime(QDate(1995, 1, 1), QTime(12, 0), lint);
+        // Trying to step into the hole gets the other side:
+        QCOMPARE(dt1.addDays(1), dt2);
+        QCOMPARE(dt2.addDays(-1), dt1);
+        // But the other side is in fact two days away:
+        QCOMPARE(dt1.addDays(2), dt2);
+        QCOMPARE(dt2.addDays(-2), dt1);
+        QCOMPARE(dt1.daysTo(dt2), 2);
+    }
+#    ifndef Q_OS_DARWIN
+    if (const QTimeZone alaska("America/Anchorage"); alaska.isValid()) {
+        // On Julian date 1867, Sat Oct 7 (at 14:31 local solar mean time for
+        // Anchorage, 15:30 LMT in Sitka, which hosted the transfer ceremony)
+        // Russia sold Alaska to the USA, which changed the calendar to
+        // Gregorian, hence the date to Fri Oct 18. Compare addSecs:Alaska-Day.
+        // Friday evening and Saturday morning were repeated, with different dates.
+        // Friday noon, as described by the Russians:
+        dt1 = QDateTime(QDate(1867, 10, 6, QCalendar(QCalendar::System::Julian)),
+                        QTime(12, 0), alaska);
+        // Sunday noon, as described by the Americans:
+        dt2 = QDateTime(QDate(1867, 10, 20), QTime(12, 0), alaska);
+        // Three elapsed days, but daysTo() and addDays only see two:
+        QCOMPARE(dt1.addDays(2), dt2);
+        QCOMPARE(dt2.addDays(-2), dt1);
+        QCOMPARE(dt1.daysTo(dt2), 2);
+        // Stepping into the duplicated day (Julian 7th, Gregorian 19th) gets
+        // the nearer side, with the same nominal date (and time):
+        QCOMPARE(dt1.addDays(1).date(), dt2.addDays(-1).date());
+        QCOMPARE(dt1.addDays(1).time(), dt2.addDays(-1).time());
+        QCOMPARE(dt1.addDays(1).daysTo(dt2.addDays(-1)), 0);
+        // Yet they differ by a day:
+        QCOMPARE_NE(dt1.addDays(1), dt2.addDays(-1));
+        QCOMPARE(dt1.addDays(1).secsTo(dt2.addDays(-1)), 24 * 60 * 60);
+        // Stepping from one duplicate one day towards the other jumps it:
+        QCOMPARE(dt1, dt2.addDays(-1).addDays(-1));
+        QCOMPARE(dt1.addDays(1).addDays(1), dt2);
+    }
+#    endif // Darwin
+#  endif // inadequate zone data
+#endif // timezone
 
     // Baja Mexico has a transition at the epoch, see fromStringDateFormat_data().
     if (QDateTime(QDate(1969, 12, 30), QTime(0, 0)).secsTo(
@@ -1578,6 +1628,43 @@ void tst_QDateTime::addMSecs_data()
     QTest::newRow("to-first")
         << QDateTime::fromSecsSinceEpoch(1 - maxSeconds, UTC) << qint64(-1)
         << QDateTime::fromSecsSinceEpoch(-maxSeconds, UTC);
+
+#if QT_CONFIG(timezone)
+    if (const QTimeZone cet("Europe/Oslo"); cet.isValid()) {
+        QTest::newRow("CET-spring-forward")
+            << QDateTime(QDate(2023, 3, 26), QTime(1, 30), cet) << qint64(60 * 60)
+            << QDateTime(QDate(2023, 3, 26), QTime(3, 30), cet);
+        QTest::newRow("CET-fall-back")
+            << QDateTime(QDate(2023, 10, 29), QTime(1, 30), cet) << qint64(3 * 60 * 60)
+            << QDateTime(QDate(2023, 10, 29), QTime(3, 30), cet);
+    }
+#  ifndef INADEQUATE_TZ_DATA
+    const QTimeZone lint("Pacific/Kiritimati");
+    if (lint.isValid()) {
+        // Line Islands Time skipped Sat 1994-12-31:
+        QTest::newRow("Kiritimati-day-off")
+            << QDateTime(QDate(1994, 12, 30), QTime(23, 30), lint) << qint64(60 * 60)
+            << QDateTime(QDate(1995, 1, 1), QTime(0, 30), lint);
+    }
+#    ifndef Q_OS_DARWIN
+    if (const QTimeZone alaska("America/Anchorage"); alaska.isValid()) {
+        // On Julian date 1867, Sat Oct 7 (at 14:31 local solar mean time for
+        // Anchorage, 15:30 LMT in Sitka, which hosted the transfer ceremony)
+        // Russia sold Alaska to the USA, which changed the calendar to
+        // Gregorian, hence the date to Fri Oct 18. Contrast addDays().
+        const QDate sat(1867, 10, 19);
+        Q_ASSERT(sat == QDate(1867, 10, 7, QCalendar(QCalendar::System::Julian)));
+        // At the start of the day, it was Sat 7th; by evening it was Fri 18th;
+        // then the next day was Sat 19th.
+        QTest::newRow("Alaska-Day")
+            // The actual morning of the hand-over:
+            << QDateTime(sat, QTime(6, 0), alaska) << qint64(12 * 60 * 60)
+            // The evening of the same day.
+            << QDateTime(sat, QTime(18, 0), alaska).addDays(-1);
+    }
+#    endif // Darwin
+#  endif // inadequate zone data
+#endif // timezone
 }
 
 void tst_QDateTime::addSecs_data()
@@ -1621,6 +1708,7 @@ void tst_QDateTime::addSecs()
         QCOMPARE(result - std::chrono::seconds(nsecs), dt);
         test3 -= std::chrono::seconds(nsecs);
         QCOMPARE(test3, dt);
+        QCOMPARE(dt.secsTo(result), nsecs);
     }
 }