diff --git a/src/corelib/time/qtimezonelocale.cpp b/src/corelib/time/qtimezonelocale.cpp
index c986b8ac280..e793a59f933 100644
--- a/src/corelib/time/qtimezonelocale.cpp
+++ b/src/corelib/time/qtimezonelocale.cpp
@@ -72,6 +72,24 @@ QString ucalTimeZoneDisplayName(UCalendar *ucal,
     return result;
 }
 
+bool ucalKnownTimeZoneId(const QString &ianaStr)
+{
+    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;
+}
+
 } // QtTimeZoneLocale
 
 // Used by TZ backends when ICU is available:
@@ -88,6 +106,11 @@ QString QTimeZonePrivate::localeName(qint64 atMSecsSinceEpoch, int offsetFromUtc
         return isoOffsetFormat(offsetFromUtc);
 
     const QString id = QString::fromUtf8(m_id);
+    // Need to check id is known to ICU, since ucal_open() will return a
+    // misleading "valid" GMT ucal when it doesn't recognise id.
+    if (!QtTimeZoneLocale::ucalKnownTimeZoneId(id))
+        return QString();
+
     const QByteArray loc = locale.name().toUtf8();
     UErrorCode status = U_ZERO_ERROR;
     UCalendar *ucal = ucal_open(reinterpret_cast<const UChar *>(id.data()), id.size(),
diff --git a/src/corelib/time/qtimezonelocale_p.h b/src/corelib/time/qtimezonelocale_p.h
index 076e29d268d..dc5de597253 100644
--- a/src/corelib/time/qtimezonelocale_p.h
+++ b/src/corelib/time/qtimezonelocale_p.h
@@ -33,6 +33,8 @@ namespace QtTimeZoneLocale {
 QString ucalTimeZoneDisplayName(UCalendar *ucal, QTimeZone::TimeType timeType,
                                 QTimeZone::NameType nameType,
                                 const QByteArray &localeCode);
+
+bool ucalKnownTimeZoneId(const QString &id);
 #else
 // Define data types for QTZL_data_p.h
 
diff --git a/src/corelib/time/qtimezoneprivate_icu.cpp b/src/corelib/time/qtimezoneprivate_icu.cpp
index c7039e157ca..ef1c83b9d05 100644
--- a/src/corelib/time/qtimezoneprivate_icu.cpp
+++ b/src/corelib/time/qtimezoneprivate_icu.cpp
@@ -387,21 +387,7 @@ QByteArray QIcuTimeZonePrivate::systemTimeZoneId() const
 
 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;
+    return QtTimeZoneLocale::ucalKnownTimeZoneId(QString::fromUtf8(ianaId));
 }
 
 QList<QByteArray> QIcuTimeZonePrivate::availableTimeZoneIds() const