diff --git a/src/corelib/global/qtenvironmentvariables.cpp b/src/corelib/global/qtenvironmentvariables.cpp
index 4f2a263cc4..6a501bb8c4 100644
--- a/src/corelib/global/qtenvironmentvariables.cpp
+++ b/src/corelib/global/qtenvironmentvariables.cpp
@@ -392,4 +392,33 @@ bool qLocalTime(time_t utc, struct tm *local)
 #endif
 }
 
+/* Access to the tzname[] global in one thread is UB if any other is calling
+   tzset() or anything that behaves as if it called tzset(). So also lock this
+   access to prevent such collisions.
+
+   Parameter dstIndex must be 1 for DST or 0 for standard time.
+   Returns the relevant form of the name of local-time's zone.
+*/
+QString qTzName(int dstIndex)
+{
+    char name[512];
+    bool ok;
+#if defined(Q_CC_MSVC)
+    size_t s = 0;
+    {
+        const auto locker = qt_scoped_lock(environmentMutex);
+        ok = _get_tzname(&s, name, 512, dstIndex) != 0;
+    }
+#else
+    {
+        const auto locker = qt_scoped_lock(environmentMutex);
+        const char *const src = tzname[dstIndex];
+        ok = src != nullptr;
+        if (ok)
+            memcpy(name, src, std::min(sizeof(name), strlen(src) + 1));
+    }
+#endif // Q_OS_WIN
+    return ok ? QString::fromLocal8Bit(name) : QString();
+}
+
 QT_END_NAMESPACE
diff --git a/src/corelib/global/qtenvironmentvariables_p.h b/src/corelib/global/qtenvironmentvariables_p.h
index afc3c5f52a..0c81d36db6 100644
--- a/src/corelib/global/qtenvironmentvariables_p.h
+++ b/src/corelib/global/qtenvironmentvariables_p.h
@@ -31,6 +31,7 @@ QT_BEGIN_NAMESPACE
 Q_CORE_EXPORT void qTzSet();
 Q_CORE_EXPORT time_t qMkTime(struct tm *when);
 Q_CORE_EXPORT bool qLocalTime(time_t utc, struct tm *local);
+Q_CORE_EXPORT QString qTzName(int dstIndex);
 
 QT_END_NAMESPACE
 
diff --git a/src/corelib/time/qdatetimeparser.cpp b/src/corelib/time/qdatetimeparser.cpp
index c4e4ddbef7..b8fbb7e03a 100644
--- a/src/corelib/time/qdatetimeparser.cpp
+++ b/src/corelib/time/qdatetimeparser.cpp
@@ -3,7 +3,6 @@
 
 #include "qplatformdefs.h"
 #include "private/qdatetimeparser_p.h"
-#include "private/qstringiterator_p.h"
 
 #include "qdatastream.h"
 #include "qdatetime.h"
@@ -13,6 +12,9 @@
 #include "qtimezone.h"
 #include "qvarlengtharray.h"
 
+#include "private/qstringiterator_p.h"
+#include "private/qtenvironmentvariables_p.h"
+
 //#define QDATETIMEPARSER_DEBUG
 #if defined (QDATETIMEPARSER_DEBUG) && !defined(QT_NO_DEBUG_STREAM)
 #  define QDTPDEBUG qDebug()
@@ -1140,6 +1142,19 @@ static QTime actualTime(QDateTimeParser::Sections known,
     return actual;
 }
 
+/*
+  \internal
+*/
+static int startsWithLocalTimeZone(QStringView name)
+{
+    for (int i = 0; i < 2; ++i) {
+        const QString zone(qTzName(i));
+        if (name.startsWith(zone))
+            return zone.size();
+    }
+    return 0;
+}
+
 /*!
   \internal
 */
diff --git a/src/corelib/time/qdatetimeparser_p.h b/src/corelib/time/qdatetimeparser_p.h
index 4c39f4b951..64ced268c9 100644
--- a/src/corelib/time/qdatetimeparser_p.h
+++ b/src/corelib/time/qdatetimeparser_p.h
@@ -177,8 +177,6 @@ private:
     ParsedSection findTimeZoneName(QStringView str, const QDateTime &when) const;
     ParsedSection findTimeZone(QStringView str, const QDateTime &when,
                                int maxVal, int minVal, int mode) const;
-    // Implemented in qlocaltime.cpp:
-    static int startsWithLocalTimeZone(const QStringView name);
 
     enum AmPmFinder {
         Neither = -1,
diff --git a/src/corelib/time/qlocaltime.cpp b/src/corelib/time/qlocaltime.cpp
index 60df487af6..3b9a236ccf 100644
--- a/src/corelib/time/qlocaltime.cpp
+++ b/src/corelib/time/qlocaltime.cpp
@@ -165,43 +165,8 @@ struct tm timeToTm(qint64 localDay, int secs, QDateTimePrivate::DaylightStatus d
     return local;
 }
 
-// Returns the tzname, assume tzset has been called already
-QString qt_tzname(QDateTimePrivate::DaylightStatus daylightStatus)
-{
-    int isDst = (daylightStatus == QDateTimePrivate::DaylightTime) ? 1 : 0;
-#if defined(Q_CC_MSVC)
-    size_t s = 0;
-    char name[512];
-    if (_get_tzname(&s, name, 512, isDst))
-        return QString();
-    return QString::fromLocal8Bit(name);
-#else
-    return QString::fromLocal8Bit(tzname[isDst]);
-#endif // Q_OS_WIN
-}
-
 } // namespace
 
-#if QT_CONFIG(datetimeparser)
-/*
-  \internal
-  Implemented here to share qt_tzname()
-*/
-int QDateTimeParser::startsWithLocalTimeZone(QStringView name)
-{
-    QDateTimePrivate::DaylightStatus zones[2] = {
-        QDateTimePrivate::StandardTime,
-        QDateTimePrivate::DaylightTime
-    };
-    for (const auto z : zones) {
-        QString zone(qt_tzname(z));
-        if (name.startsWith(zone))
-            return zone.size();
-    }
-    return 0;
-}
-#endif // datetimeparser
-
 namespace QLocalTime {
 
 #ifndef QT_BOOTSTRAPPED
@@ -319,8 +284,7 @@ QString localTimeAbbbreviationAt(qint64 local, QDateTimePrivate::DaylightStatus
     time_t utcSecs;
     if (!callMkTime(&tmLocal, &utcSecs))
         return {};
-    return qt_tzname(tmLocal.tm_isdst > 0 ? QDateTimePrivate::DaylightTime
-                                          : QDateTimePrivate::StandardTime);
+    return qTzName(tmLocal.tm_isdst > 0 ? 1 : 0);
 }
 
 QDateTimePrivate::ZoneState mapLocalTime(qint64 local, QDateTimePrivate::DaylightStatus dst)