diff --git a/src/network/access/qnetworkrequest.cpp b/src/network/access/qnetworkrequest.cpp
index 64d35aa2782..de40b2c088e 100644
--- a/src/network/access/qnetworkrequest.cpp
+++ b/src/network/access/qnetworkrequest.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2022 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtNetwork module of the Qt Toolkit.
@@ -1071,9 +1071,10 @@ static QByteArray headerValue(QNetworkRequest::KnownHeaders header, const QVaria
     case QNetworkRequest::LastModifiedHeader:
     case QNetworkRequest::IfModifiedSinceHeader:
         switch (value.userType()) {
+            // Generate RFC 1123/822 dates:
         case QMetaType::QDate:
+            return QNetworkHeadersPrivate::toHttpDate(value.toDate().startOfDay(Qt::UTC));
         case QMetaType::QDateTime:
-            // generate RFC 1123/822 dates:
             return QNetworkHeadersPrivate::toHttpDate(value.toDateTime());
 
         default:
@@ -1515,8 +1516,7 @@ QDateTime QNetworkHeadersPrivate::fromHttpDate(const QByteArray &value)
 
 QByteArray QNetworkHeadersPrivate::toHttpDate(const QDateTime &dt)
 {
-    return QLocale::c().toString(dt, u"ddd, dd MMM yyyy hh:mm:ss 'GMT'")
-        .toLatin1();
+    return QLocale::c().toString(dt.toUTC(), u"ddd, dd MMM yyyy hh:mm:ss 'GMT'").toLatin1();
 }
 
 QT_END_NAMESPACE
diff --git a/tests/auto/network/access/qnetworkrequest/tst_qnetworkrequest.cpp b/tests/auto/network/access/qnetworkrequest/tst_qnetworkrequest.cpp
index d90289ea08e..8cd37a21c72 100644
--- a/tests/auto/network/access/qnetworkrequest/tst_qnetworkrequest.cpp
+++ b/tests/auto/network/access/qnetworkrequest/tst_qnetworkrequest.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2022 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the test suite of the Qt Toolkit.
@@ -26,9 +26,11 @@
 **
 ****************************************************************************/
 
-
 #include <QTest>
 #include <QtCore/QUrl>
+#if QT_CONFIG(timezone)
+#  include <QtCore/QTimeZone>
+#endif
 #include <QtNetwork/QNetworkRequest>
 #include <QtNetwork/QNetworkCookie>
 
@@ -236,12 +238,28 @@ void tst_QNetworkRequest::setHeader_data()
                                         << QVariant(QDate(2007, 11, 01))
                                         << true << "Last-Modified"
                                         << "Thu, 01 Nov 2007 00:00:00 GMT";
-    QTest::newRow("Last-Modified-DateTime") << QNetworkRequest::LastModifiedHeader
-                                            << QVariant(QDateTime(QDate(2007, 11, 01),
-                                                                  QTime(18, 8, 30),
-                                                                  Qt::UTC))
-                                            << true << "Last-Modified"
-                                            << "Thu, 01 Nov 2007 18:08:30 GMT";
+    QTest::newRow("Last-Modified-DateTime-UTC")
+        << QNetworkRequest::LastModifiedHeader
+        << QVariant(QDateTime(QDate(2007, 11, 1), QTime(18, 8, 30), Qt::UTC))
+        << true << "Last-Modified" << "Thu, 01 Nov 2007 18:08:30 GMT";
+    // QTBUG-80666: format dates correctly (as GMT) even if the date passed in isn't in UTC:
+    QTest::newRow("Last-Modified-DateTime-Local")
+        << QNetworkRequest::LastModifiedHeader
+        << QVariant(QDateTime(QDate(2007, 11, 1), QTime(18, 8, 30), Qt::UTC).toLocalTime())
+        << true << "Last-Modified" << "Thu, 01 Nov 2007 18:08:30 GMT";
+    QTest::newRow("Last-Modified-DateTime-Offset")
+        << QNetworkRequest::LastModifiedHeader
+        << QVariant(QDateTime(QDate(2007, 11, 1), QTime(18, 8, 30), Qt::UTC).toOffsetFromUtc(3600))
+        << true << "Last-Modified" << "Thu, 01 Nov 2007 18:08:30 GMT";
+#if QT_CONFIG(timezone)
+    QTimeZone cet("Europe/Oslo");
+    if (cet.isValid()) {
+        QTest::newRow("Last-Modified-DateTime-CET")
+            << QNetworkRequest::LastModifiedHeader
+            << QVariant(QDateTime(QDate(2007, 11, 1), QTime(18, 8, 30), Qt::UTC).toTimeZone(cet))
+            << true << "Last-Modified" << "Thu, 01 Nov 2007 18:08:30 GMT";
+    }
+#endif
 
     QTest::newRow("If-Modified-Since-Date") << QNetworkRequest::IfModifiedSinceHeader
                                         << QVariant(QDate(2017, 7, 01))