additional check for presenter

src/Avalonia.Controls/DateTimePickers/DatePickerPresenter.cs

@@ -348,8 +348,7 @@ namespace Avalonia.Controls
             var dt = Date;
             if (DayVisible)
             {
-                GregorianCalendar gc = new GregorianCalendar();
-                var maxDays = gc.GetDaysInMonth(dt.Year, dt.Month);
+                var maxDays = _calendar.GetDaysInMonth(dt.Year, dt.Month);
                 _daySelector.MaximumValue = maxDays;
                 _daySelector.MinimumValue = 1;
                 _daySelector.SelectedValue = dt.Day;
@@ -407,10 +406,14 @@ namespace Avalonia.Controls
                 }
             }
 
-            Grid.SetColumn(_spacer1, 1);
-            Grid.SetColumn(_spacer2, 3);
-            _spacer1.IsVisible = columnIndex > 1;
-            _spacer2.IsVisible = columnIndex > 2;
+            var isSpacer1Visible = columnIndex > 1;
+            var isSpacer2Visible = columnIndex > 2;
+            // ternary conditional operator is used to make sure grid cells will be validated
+            Grid.SetColumn(_spacer1, isSpacer1Visible ? 1 : 0);
+            Grid.SetColumn(_spacer2, isSpacer2Visible ? 3 : 0);
+
+            _spacer1.IsVisible = isSpacer1Visible;
+            _spacer2.IsVisible = isSpacer2Visible;
         }
 
         private void SetInitialFocus()