diff --git a/src/corelib/time/qtimezoneprivate_tz.cpp b/src/corelib/time/qtimezoneprivate_tz.cpp
index 2955a5c957..8f02e3a7d6 100644
--- a/src/corelib/time/qtimezoneprivate_tz.cpp
+++ b/src/corelib/time/qtimezoneprivate_tz.cpp
@@ -393,19 +393,22 @@ static QDate calculatePosixDate(const QByteArray &dateRule, int year)
                 return calculateDowDate(year, month, dow, week);
         }
     } else if (dateRule.at(0) == 'J') {
-        // Day of Year ignores Feb 29
+        // Day of Year 1...365, ignores Feb 29.
+        // So March always starts on day 60.
         int doy = dateRule.mid(1).toInt(&ok);
         if (ok && doy > 0 && doy < 366) {
-            QDate date = QDate(year, 1, 1).addDays(doy - 1);
-            if (QDate::isLeapYear(date.year()) && date.month() > 2)
-                date = date.addDays(-1);
-            return date;
+            // Subtract 1 because we're adding days *after* the first of
+            // January, unless it's after February in a leap year, when the leap
+            // day cancels that out:
+            if (!QDate::isLeapYear(year) || doy < 60)
+                --doy;
+            return QDate(year, 1, 1).addDays(doy);
         }
     } else {
-        // Day of Year includes Feb 29
+        // Day of Year 0...365, includes Feb 29
         int doy = dateRule.toInt(&ok);
-        if (ok && doy > 0 && doy <= 366)
-            return QDate(year, 1, 1).addDays(doy - 1);
+        if (ok && doy >= 0 && doy < 366)
+            return QDate(year, 1, 1).addDays(doy);
     }
     return QDate();
 }
diff --git a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
index 2e197cca4a..19b638cce7 100644
--- a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
+++ b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
@@ -1141,6 +1141,18 @@ void tst_QTimeZone::tzTest()
     QTzTimeZonePrivate tzposix("MET-1METDST-2,M3.5.0/02:00:00,M10.5.0/03:00:00");
     QVERIFY(tzposix.isValid());
 
+    // RHEL has been seen with this as Africa/Casablanca's POSIX rule:
+    QTzTimeZonePrivate permaDst("<+00>0<+01>,0/0,J365/25");
+    const QTimeZone utcP1("UTC+01:00"); // Should always have same offset as permaDst
+    QVERIFY(permaDst.isValid());
+    QVERIFY(permaDst.hasDaylightTime());
+    QVERIFY(permaDst.isDaylightTime(QDate(2020, 1, 1).startOfDay(utcP1).toMSecsSinceEpoch()));
+    QVERIFY(permaDst.isDaylightTime(QDate(2020, 12, 31).endOfDay(utcP1).toMSecsSinceEpoch()));
+    // Note that the final /25 could be misunderstood as putting a fall-back at
+    // 1am on the next year's Jan 1st; check we don't do that:
+    QVERIFY(permaDst.isDaylightTime(
+                QDateTime(QDate(2020, 1, 1), QTime(1, 30), utcP1).toMSecsSinceEpoch()));
+
     QTimeZone tzBrazil("BRT+3"); // parts of Northern Brazil, as a POSIX rule
     QVERIFY(tzBrazil.isValid());
     QCOMPARE(tzBrazil.offsetFromUtc(QDateTime(QDate(1111, 11, 11).startOfDay())), -10800);