diff --git a/src/corelib/time/qtimezone.cpp b/src/corelib/time/qtimezone.cpp
index 6713bd0d48d..12c53ce024e 100644
--- a/src/corelib/time/qtimezone.cpp
+++ b/src/corelib/time/qtimezone.cpp
@@ -465,24 +465,15 @@ QTimeZone::Data &QTimeZone::Data::operator=(QTimeZonePrivate *dptr) noexcept
 
 QTimeZone::QTimeZone(const QByteArray &ianaId)
 {
-    // Try and see if it's a CLDR UTC offset ID - just as quick by creating as
-    // by looking up.
+    // Try and see if it's a recognized UTC offset ID - just as quick by
+    // creating as by looking up.
     d = new QUtcTimeZonePrivate(ianaId);
-    // If not a CLDR UTC offset ID then try creating it with the system backend.
-    // Relies on backend not creating valid TZ with invalid name.
+    // If not recognized, try creating it with the system backend.
     if (!d->isValid()) {
         if (ianaId.isEmpty())
             d = newBackendTimeZone();
-#ifdef Q_OS_ANDROID
-        // on Android the isTimeZoneIdAvailable() implementation is vastly more
-        // expensive than just trying to create a timezone
-        else
+        else // Constructor MUST produce invalid for unsupported ID.
             d = newBackendTimeZone(ianaId);
-#else
-        else if (global_tz->backend->isTimeZoneIdAvailable(ianaId))
-            d = newBackendTimeZone(ianaId);
-#endif
-        // else: No such ID, avoid creating a TZ cache entry for it.
     }
     // Can also handle UTC with arbitrary (valid) offset, but only do so as
     // fall-back, since either of the above may handle it more informatively.
diff --git a/src/corelib/time/qtimezoneprivate_icu.cpp b/src/corelib/time/qtimezoneprivate_icu.cpp
index c071e7d549d..d2f91d35e20 100644
--- a/src/corelib/time/qtimezoneprivate_icu.cpp
+++ b/src/corelib/time/qtimezoneprivate_icu.cpp
@@ -254,8 +254,8 @@ QIcuTimeZonePrivate::QIcuTimeZonePrivate()
 QIcuTimeZonePrivate::QIcuTimeZonePrivate(const QByteArray &ianaId)
     : m_ucal(nullptr)
 {
-    // Need to check validity here as ICu will create a GMT tz if name is invalid
-    if (availableTimeZoneIds().contains(ianaId))
+    // ICU misleadingly maps invalid IDs to GMT.
+    if (isTimeZoneIdAvailable(ianaId))
         init(ianaId);
 }
 
@@ -442,6 +442,25 @@ QByteArray QIcuTimeZonePrivate::systemTimeZoneId() const
     return ucalDefaultTimeZoneId();
 }
 
+bool QIcuTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray &ianaId) const
+{
+    const QString ianaStr = QString::fromUtf8(ianaId);
+    const UChar *const name = reinterpret_cast<const UChar *>(ianaStr.constData());
+    // We are not interested in the value, but we have to pass something.
+    // No known IANA zone name is (up to 2023) longer than 30 characters.
+    constexpr size_t size = 64;
+    UChar buffer[size];
+
+    // TODO: convert to ucal_getIanaTimeZoneID(), new draft in ICU 74, once we
+    // can rely on its availability, assuming it works the same once not draft.
+    UErrorCode status = U_ZERO_ERROR;
+    UBool isSys = false;
+    // Returns the length of the IANA zone name (but we don't care):
+    ucal_getCanonicalTimeZoneID(name, ianaStr.size(), buffer, size, &isSys, &status);
+    // We're only interested if the result is a "system" (i.e. IANA) ID:
+    return isSys;
+}
+
 QList<QByteArray> QIcuTimeZonePrivate::availableTimeZoneIds() const
 {
     UErrorCode status = U_ZERO_ERROR;
diff --git a/src/corelib/time/qtimezoneprivate_p.h b/src/corelib/time/qtimezoneprivate_p.h
index f04157a654c..297e62c7781 100644
--- a/src/corelib/time/qtimezoneprivate_p.h
+++ b/src/corelib/time/qtimezoneprivate_p.h
@@ -233,6 +233,7 @@ public:
 
     QByteArray systemTimeZoneId() const override;
 
+    bool isTimeZoneIdAvailable(const QByteArray &ianaId) const override;
     QList<QByteArray> availableTimeZoneIds() const override;
     QList<QByteArray> availableTimeZoneIds(QLocale::Territory territory) const override;
     QList<QByteArray> availableTimeZoneIds(int offsetFromUtc) const override;
diff --git a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
index 13a736a3281..09c80642a94 100644
--- a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
+++ b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
@@ -1281,7 +1281,9 @@ void tst_QTimeZone::icuTest()
     QIcuTimeZonePrivate tzpd;
     QVERIFY(tzpd.isValid());
 
-    // Test invalid constructor
+    // Test invalid is not available:
+    QVERIFY(!tzpd.isTimeZoneIdAvailable("Gondwana/Erewhon"));
+    // and construction gives an invalid result:
     QIcuTimeZonePrivate tzpi("Gondwana/Erewhon");
     QCOMPARE(tzpi.isValid(), false);