diff --git a/src/corelib/time/qtimezone.cpp b/src/corelib/time/qtimezone.cpp
index 85989154e79..4976ea5fcb8 100644
--- a/src/corelib/time/qtimezone.cpp
+++ b/src/corelib/time/qtimezone.cpp
@@ -532,7 +532,9 @@ QTimeZone::QTimeZone(int offsetSeconds)
 
 QTimeZone::QTimeZone(const QByteArray &zoneId, int offsetSeconds, const QString &name,
                      const QString &abbreviation, QLocale::Territory territory, const QString &comment)
-    : d(isTimeZoneIdAvailable(zoneId) ? nullptr // Don't let client code hijack a real zone name.
+    : d(QUtcTimeZonePrivate().isTimeZoneIdAvailable(zoneId)
+        || global_tz->backend->isTimeZoneIdAvailable(zoneId)
+        ? nullptr // Don't let client code hijack a real zone name.
         : new QUtcTimeZonePrivate(zoneId, offsetSeconds, name, abbreviation, territory, comment))
 {
 }
@@ -1420,6 +1422,7 @@ bool QTimeZone::isTimeZoneIdAvailable(const QByteArray &ianaId)
         return false;
 #endif
     return QUtcTimeZonePrivate().isTimeZoneIdAvailable(ianaId)
+        || QUtcTimeZonePrivate::offsetFromUtcString(ianaId) != QTimeZonePrivate::invalidSeconds()
         || global_tz->backend->isTimeZoneIdAvailable(ianaId);
 }
 
diff --git a/src/corelib/time/qtimezoneprivate.cpp b/src/corelib/time/qtimezoneprivate.cpp
index f007b05d58f..3a9bc695cbf 100644
--- a/src/corelib/time/qtimezoneprivate.cpp
+++ b/src/corelib/time/qtimezoneprivate.cpp
@@ -919,7 +919,9 @@ bool QUtcTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray &ianaId) const
         if (isEntryInIanaList(ianaId, data.id()))
             return true;
     }
-    // But see offsetFromUtcString(), which lets us accept some "unavailable" IDs.
+    // Callers may want to || offsetFromUtcString(ianaId) != invalidSeconds(),
+    // but those are technically not IANA IDs and the custom QTimeZone
+    // constructor needs the return here to reflect that.
     return false;
 }
 
diff --git a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
index ff6f96c1a05..29643b2caca 100644
--- a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
+++ b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
@@ -538,6 +538,12 @@ void tst_QTimeZone::isTimeZoneIdAvailable()
         QVERIFY2(QTimeZone::isTimeZoneIdAvailable(id), id);
         QVERIFY2(QTimeZone(id).isValid(), id);
     }
+    for (qint32 offset = QTimeZone::MinUtcOffsetSecs;
+         offset <= QTimeZone::MinUtcOffsetSecs; ++offset) {
+        const QByteArray id = QTimeZone(offset).id();
+        QVERIFY2(QTimeZone::isTimeZoneIdAvailable(id), id);
+        QVERIFY2(QTimeZone(id).isValid(), id);
+    }
 }
 
 void tst_QTimeZone::utcOffsetId_data()