Fix scroll index.

public int trueIndexOf(int index)  : in flutter Example: [ListWheelChildLoopingListDelegate] is built by looping a list of
  /// length 8. Then, trueIndexOf(10) = 2 and trueIndexOf(-5) = 3.
new DateTime(year: selectedYear, month: selectedMonth,
                                day: selectedDay)) : like in february, the date is still in 30, which will cause errors.

com.unity.uiwidgets/Runtime/cupertino/date_picker.cs

         bool _isCurrentDateValid {
             // The current date selection represents a range [minSelectedData, maxSelectDate].
             get {
+               
                 var minSelectedDate = new DateTime(year: selectedYear, month: selectedMonth, 1);
                 minSelectedDate = minSelectedDate.AddDays(selectedDay - 1);
                 var maxSelectedDate = minSelectedDate.AddDays(1);
                     onSelectedItemChanged: index => {
                         selectedDay = index + 1;
                         if (_isCurrentDateValid) {
-                            widget.onDateTimeChanged(new DateTime(year: selectedYear, month: selectedMonth,
-                                day: selectedDay));
+                            var date = new DateTime(year: selectedYear, month: selectedMonth,day:1);
+                            date.AddDays(selectedDay - 1);
+                            widget.onDateTimeChanged(date);
                         }
                     },
                     children: widgets,
                     backgroundColor: widget.backgroundColor,
                     squeeze: CupertinoDatePickerUtils._kSqueeze,
                     onSelectedItemChanged: index => {
-                        if (index < 0) {
-                            index = -index;
-                        }
-
-                            widget.onDateTimeChanged(new DateTime(year: selectedYear, month: selectedMonth,
-                                day: selectedDay));
+                            var date = new DateTime(year: selectedYear, month: selectedMonth,day:1);
+                            date.AddDays(selectedDay - 1);
+                            widget.onDateTimeChanged(date);
                         }
                     },
                     children: widgets,
                     onSelectedItemChanged: index => {
                         selectedYear = index;
                         if (_isCurrentDateValid) {
-                            widget.onDateTimeChanged(new DateTime(year: selectedYear, month: selectedMonth,
-                                day: selectedDay));
+                            var date = new DateTime(year: selectedYear, month: selectedMonth,day:1);
+                            date.AddDays(selectedDay - 1);
+                            widget.onDateTimeChanged(date);
                         }
                     },
                     itemBuilder: (_context, year) => {

com.unity.uiwidgets/Runtime/widgets/list_wheel_scroll_view.cs

         }
 
         public int trueIndexOf(int index) {
-            
-            return index % children.Count;
+            var result = index;
+            if (index < 0) {
+                if (index % children.Count == 0)
+                    result = 0;
+                else {
+                    result = index % children.Count + children.Count;
+                }
+            }
+            else 
+                result = index % children.Count;
+            return result;
         }
 
         public Widget build(BuildContext context, int index) {