backport fix date/time display after timezone changes

one has to instruct the JS QML engine that the local timezone
changed (e.g. after editting it in kcm clock); keep track of the tzOffset
and do a recalc if it changes

applets/digital-clock/package/contents/ui/DigitalClock.qml

@@ -18,7 +18,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-import QtQuick 2.0
+import QtQuick 2.2
 import QtQuick.Layouts 1.1
 import org.kde.plasma.core 2.0 as PlasmaCore
 import org.kde.plasma.components 2.0 as Components
@@ -45,6 +45,7 @@ Item {
     property bool showSeconds: plasmoid.configuration.showSeconds
     property bool showTimezone: plasmoid.configuration.showTimezone
     property string timeFormat
+    property int tzOffset
 
     onShowSecondsChanged: {
         timeFormatCorrection(Qt.locale().timeFormat(Locale.ShortFormat))
@@ -147,11 +148,31 @@ Item {
             timeFormatString = timeFormatString + " t";
         }
 
-
         main.timeFormat = timeFormatString;
     }
 
+    function dateTimeChanged()
+    {
+        //console.log("Date/time changed!");
+        var doCorrections = false;
+
+        var currentTZOffset = dataSource.data["Local"]["Offset"] / 60;
+        if (currentTZOffset != tzOffset) {
+            doCorrections = true;
+            tzOffset = currentTZOffset;
+            //console.log("TZ offset changed: " + tzOffset);
+            Date.timeZoneUpdated(); // inform the QML JS engine about TZ change
+        }
+
+        if (doCorrections) {
+            timeFormatCorrection(main.timeFormat);
+        }
+    }
+
     Component.onCompleted: {
-        timeFormatCorrection(Qt.locale().timeFormat(Locale.ShortFormat))
+        tzOffset = new Date().getTimezoneOffset();
+        //console.log("Initial TZ offset: " + tzOffset);
+        timeFormatCorrection(Qt.locale().timeFormat(Locale.ShortFormat));
+        dataSource.onDataChanged.connect(dateTimeChanged);
     }
 }