U4-516 - Fix DatePicker with time default value

Enables the standard jQuery date picker to parse the current value of
the field correctly, this prevents it from defaulting to Today and
overwriting the currently stored value.

src/Umbraco.Web.UI/umbraco_client/DateTimePicker/timepicker.js

@@ -62,9 +62,19 @@ $.datepicker._connectDatepicker = function(target, inst) {
  */
 $.datepicker._showDatepickerOverride = $.datepicker._showDatepicker;
 $.datepicker._showDatepicker = function (input) {
+    // keep the current value
+    var originalval = input.value;
+
+    // Keep the first 10 chars for now yyyy-mm-dd - this removes the time part which was breaking the standardDatePicker parsing code
+    input.value = originalval.length>10 ? originalval.substring(0, 10) : originalval;
+
+
     // Call the original method which will show the datepicker
     $.datepicker._showDatepickerOverride(input);
 
+    // Put it back
+    input.value = originalval;
+
     input = input.target || input;
 
     // find from button/image trigger