diff --git a/src/corelib/time/qtimezone.cpp b/src/corelib/time/qtimezone.cpp
index 45e37cb4031..e95700c64b8 100644
--- a/src/corelib/time/qtimezone.cpp
+++ b/src/corelib/time/qtimezone.cpp
@@ -79,6 +79,7 @@ static QTimeZonePrivate *newBackendTimeZone()
 // Create named time zone using appropriate backend
 static QTimeZonePrivate *newBackendTimeZone(const QByteArray &ianaId)
 {
+    Q_ASSERT(!ianaId.isEmpty());
 #ifdef QT_NO_SYSTEMLOCALE
 #if QT_CONFIG(icu)
     return new QIcuTimeZonePrivate(ianaId);
@@ -339,7 +340,7 @@ QTimeZone::QTimeZone(const QByteArray &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 (!d->isValid())
-        d = newBackendTimeZone(ianaId);
+        d = ianaId.isEmpty() ? newBackendTimeZone() : newBackendTimeZone(ianaId);
     // 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.
     if (!d->isValid()) {
@@ -767,7 +768,15 @@ QTimeZone::OffsetDataList QTimeZone::transitions(const QDateTime &fromDateTime,
 
 QByteArray QTimeZone::systemTimeZoneId()
 {
-    return global_tz->backend->systemTimeZoneId();
+    const QByteArray sys = global_tz->backend->systemTimeZoneId();
+    if (!sys.isEmpty())
+        return sys;
+    // The system zone, despite the empty ID, may know its real ID anyway:
+    auto zone = systemTimeZone();
+    if (zone.isValid() && !zone.id().isEmpty())
+        return zone.id();
+    // If all else fails, guess UTC.
+    return QTimeZonePrivate::utcQByteArray();
 }
 
 /*!
@@ -779,7 +788,7 @@ QByteArray QTimeZone::systemTimeZoneId()
 */
 QTimeZone QTimeZone::systemTimeZone()
 {
-    return QTimeZone(QTimeZone::systemTimeZoneId());
+    return QTimeZone(global_tz->backend->systemTimeZoneId());
 }
 
 /*!
diff --git a/src/corelib/time/qtimezoneprivate_tz.cpp b/src/corelib/time/qtimezoneprivate_tz.cpp
index 0ac419ba5ad..4ac4559d368 100644
--- a/src/corelib/time/qtimezoneprivate_tz.cpp
+++ b/src/corelib/time/qtimezoneprivate_tz.cpp
@@ -641,7 +641,7 @@ QTzTimeZonePrivate::QTzTimeZonePrivate()
 // Create a named time zone
 QTzTimeZonePrivate::QTzTimeZonePrivate(const QByteArray &ianaId)
 {
-    init(ianaId.isEmpty() ? systemTimeZoneId() : ianaId);
+    init(ianaId);
 }
 
 QTzTimeZonePrivate::~QTzTimeZonePrivate()
@@ -854,9 +854,6 @@ QTzTimeZoneCacheEntry QTzTimeZoneCache::fetchEntry(const QByteArray &ianaId)
 
 void QTzTimeZonePrivate::init(const QByteArray &ianaId)
 {
-    // System ID defaults to UTC, so is never empty; and our callers default to
-    // the system ID if what they're given is empty.
-    Q_ASSERT(!ianaId.isEmpty());
     static QTzTimeZoneCache tzCache;
     const auto &entry = tzCache.fetchEntry(ianaId);
     if (entry.m_tranTimes.isEmpty() && entry.m_posixRule.isEmpty())
@@ -864,6 +861,15 @@ void QTzTimeZonePrivate::init(const QByteArray &ianaId)
 
     cached_data = std::move(entry);
     m_id = ianaId;
+    // Avoid empty ID, if we have an abbreviation to use instead
+    if (m_id.isEmpty()) { // We've read /etc/localtime's contents
+        for (const auto &abbr : cached_data.m_abbreviations) {
+            if (!abbr.isEmpty()) {
+                m_id = abbr;
+                break;
+            }
+        }
+    }
 }
 
 QLocale::Country QTzTimeZonePrivate::country() const
@@ -1261,10 +1267,6 @@ QByteArray QTzTimeZonePrivate::systemTimeZoneId() const
         ianaId = reader.name();
     }
 
-    // Give up for now and return UTC
-    if (ianaId.isEmpty())
-        ianaId = utcQByteArray();
-
     return ianaId;
 }
 
diff --git a/src/corelib/time/qtimezoneprivate_win.cpp b/src/corelib/time/qtimezoneprivate_win.cpp
index 1dc362e45a8..7c80dbc38bb 100644
--- a/src/corelib/time/qtimezoneprivate_win.cpp
+++ b/src/corelib/time/qtimezoneprivate_win.cpp
@@ -510,6 +510,8 @@ void QWinTimeZonePrivate::init(const QByteArray &ianaId)
         m_id.clear();
         m_windowsId.clear();
         m_displayName.clear();
+    } else if (m_id.isEmpty()) {
+        m_id = m_standardName.toUtf8();
     }
 }
 
@@ -732,12 +734,8 @@ QByteArray QWinTimeZonePrivate::systemTimeZoneId() const
     if (country != QLocale::AnyCountry)
         ianaId = windowsIdToDefaultIanaId(windowsId, country);
     // If we don't have a real country, or there wasn't a specific match, try the global default
-    if (ianaId.isEmpty()) {
+    if (ianaId.isEmpty())
         ianaId = windowsIdToDefaultIanaId(windowsId);
-        // If no global default then probably an unknown Windows ID so return UTC
-        if (ianaId.isEmpty())
-            return utcQByteArray();
-    }
     return ianaId;
 }