Add QTimeZone::{systemTimeZone,utc}

[ChangeLog][QtCore][QTimeZone] Added methods systemTimeZone() and utc()
that return QTimeZone objects for the system time zone and for UTC,
respectively.

Change-Id: Ic5d393bfd36e48a193fcffff13b96821bb8514b9
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/tools/qdatetime.cpp

@@ -3186,13 +3186,11 @@ QTimeZone QDateTime::timeZone() const
             d->m_timeZone = QTimeZone(d->m_offsetFromUtc);
         return d->m_timeZone;
     case Qt::UTC:
-        if (!d->m_timeZone.isValid())
-            d->m_timeZone = QTimeZone(QTimeZonePrivate::utcQByteArray());
-        return d->m_timeZone;
-    case Qt::TimeZone :
+        return QTimeZone::utc();
+    case Qt::TimeZone:
         return d->m_timeZone;
     case Qt::LocalTime:
-        return QTimeZone(QTimeZone::systemTimeZoneId());
+        return QTimeZone::systemTimeZone();
     }
     return QTimeZone();
 }

src/corelib/tools/qtimezone.cpp

@@ -788,6 +788,29 @@ QByteArray QTimeZone::systemTimeZoneId()
     return global_tz->backend->systemTimeZoneId();
 }
 
+/*!
+    \since 5.5
+    Returns a QTimeZone object that refers to the local system time, as
+    specified by systemTimeZoneId().
+
+    \sa utc()
+*/
+QTimeZone QTimeZone::systemTimeZone()
+{
+    return QTimeZone(QTimeZone::systemTimeZoneId());
+}
+
+/*!
+    \since 5.5
+    Returns a QTimeZone object that refers to UTC (Universal Time Coordinated).
+
+    \sa systemTimeZone()
+*/
+QTimeZone QTimeZone::utc()
+{
+    return QTimeZone(QTimeZonePrivate::utcQByteArray());
+}
+
 /*!
     Returns \c true if a given time zone \a ianaId is available on this system.
 

src/corelib/tools/qtimezone.h

@@ -117,6 +117,8 @@ public:
     OffsetDataList transitions(const QDateTime &fromDateTime, const QDateTime &toDateTime) const;
 
     static QByteArray systemTimeZoneId();
+    static QTimeZone systemTimeZone();
+    static QTimeZone utc();
 
     static bool isTimeZoneIdAvailable(const QByteArray &ianaId);