[Windows]Fix Dot based Date Format property in DatePicker not working on Windows

src/Controls/tests/TestCases.HostApp/Issues/Issue10805.cs

@@ -0,0 +1,29 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 10805, "Date Format property in DatePicker not working on Windows with date Format", PlatformAffected.UWP)]
+public class Issue10805 : ContentPage
+{
+	public Issue10805()
+	{
+		var label = new Label
+		{
+			Text = "Verify DatePicker DateFormat",
+			AutomationId = "Label",
+		};
+
+		var datePicker = new DatePicker
+		{
+			Format = "dd.MM.yyyy",
+			Date = new DateTime(2022, 2, 25),
+		};
+
+		Content = new StackLayout
+		{
+			Children =
+			{
+				label,
+				datePicker
+			},
+		};
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue10805.cs

@@ -0,0 +1,23 @@
+#if TEST_FAILS_ON_CATALYST  //More Info: https://github.com/dotnet/maui/issues/29099
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+public class Issue10805 : _IssuesUITest
+{
+	public Issue10805(TestDevice device) : base(device)
+	{
+	}
+
+	public override string Issue => "Date Format property in DatePicker not working on Windows with date Format";
+
+	[Test]
+	[Category(UITestCategories.DatePicker)]
+	public void VerifyDatePickerDotBasedDateFormat()
+	{
+		App.WaitForElement("Label");
+		VerifyScreenshot();
+	}
+}
+#endif

src/Core/src/Platform/Windows/CalendarDatePickerExtensions.cs

@@ -43,6 +43,8 @@ internal static string GetSeparator(string format)
 				separator = "-";
 			else if (format.Contains(' ', StringComparison.CurrentCultureIgnoreCase))
 				separator = " ";
+			else if (format.Contains('.', StringComparison.CurrentCultureIgnoreCase))
+				separator = ".";
 			else
 				separator = string.Empty;