[iOS] Fix DatePicker to consistently display 4-digit years

src/Controls/tests/TestCases.HostApp/Issues/Issue31167.xaml

@@ -0,0 +1,45 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue31167">
+  <VerticalStackLayout
+    Spacing="20"
+    Padding="20">
+
+    <Label 
+      Text="DatePicker Format Test"
+      FontSize="18"
+      FontAttributes="Bold"
+      HorizontalOptions="Center" />
+
+    <Label 
+      Text="This sample tests the 4-digit year format consistency on iOS DatePicker controls."
+      HorizontalOptions="Center" />
+
+    <DatePicker 
+      x:Name="MyDatePicker"
+      AutomationId="MyDatePicker"
+      HorizontalOptions="Center"
+      Format="d" />
+
+    <Entry 
+      x:Name="DateEntry"
+      AutomationId="DateEntry"
+      Placeholder="Enter date (e.g., 12/24/2024)"
+      HorizontalOptions="FillAndExpand" />
+
+    <Button 
+      Text="Set Date"
+      AutomationId="SetDateButton"
+      Clicked="OnSetDateClicked"
+      HorizontalOptions="Center" />
+
+    <Label 
+      x:Name="DisplayLabel"
+      AutomationId="DisplayLabel"
+      Text=""
+      HorizontalOptions="Center"
+      FontAttributes="Bold" />
+
+  </VerticalStackLayout>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue31167.xaml.cs

@@ -0,0 +1,29 @@
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 31167,
+		"DatePicker initially shows year in 4 digits, but after changing the year it displays only 2 digits in net 10.0",
+		PlatformAffected.iOS)]
+	public partial class Issue31167 : ContentPage
+	{
+		public Issue31167()
+		{
+			InitializeComponent();
+			MyDatePicker.Date = new DateTime(2024, 12, 23);
+			UpdateDisplayLabel();
+		}
+
+		void OnSetDateClicked(object sender, EventArgs e)
+		{
+			if (DateTime.TryParse(DateEntry.Text, out DateTime newDate))
+			{
+				MyDatePicker.Date = newDate;
+				UpdateDisplayLabel();
+			}
+		}
+
+		void UpdateDisplayLabel()
+		{
+			DisplayLabel.Text = $"Current Date: {MyDatePicker.Date:d}";
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue31167.cs

@@ -0,0 +1,126 @@
+#if IOS
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue31167 : _IssuesUITest
+{
+	public Issue31167(TestDevice testDevice) : base(testDevice)
+	{
+	}
+
+	public override string Issue => "DatePicker initially shows year in 4 digits, but after changing the year it displays only 2 digits in net 10.0";
+
+	[Test, Order(1)]
+	[Category(UITestCategories.DatePicker)]
+	public void DatePickerYearFormat_InitialState_DisplaysFourDigitYear()
+	{
+		// Verify initial display shows 4-digit year for 2024-12-23
+		var datePickerText = App.WaitForElement("MyDatePicker").GetText();
+		Assert.That(datePickerText, Does.Contain("2024"), "DatePicker should display 4-digit year initially");
+
+		var displayLabelText = App.WaitForElement("DisplayLabel").GetText();
+		Assert.That(displayLabelText, Does.Contain("2024"), "Display label should show 4-digit year");
+	}
+
+	[Test, Order(2)]
+	[Category(UITestCategories.DatePicker)]
+	public void DatePickerYearFormat_UserInteraction_MaintainsFourDigitYear()
+	{
+		// Get initial text
+		var initialText = App.WaitForElement("MyDatePicker").GetText();
+		Assert.That(initialText, Does.Contain("2024"), "Initial display should show 4-digit year");
+
+		// Simulate user interaction by opening and closing the DatePicker
+		App.Tap("MyDatePicker");
+
+		// On iOS, tap Done to close the picker
+		App.WaitForElement("Done");
+		App.Tap("Done");
+
+		// Verify text still shows 4-digit year after interaction
+		var afterInteractionText = App.WaitForElement("MyDatePicker").GetText();
+		Assert.That(afterInteractionText, Does.Contain("2024"), "After interaction should still show 4-digit year");
+
+		// Ensure format consistency
+		Assert.That(afterInteractionText, Is.EqualTo(initialText),
+			"Date format should remain consistent after user interaction");
+	}
+
+	[Test, Order(3)]
+	[Category(UITestCategories.DatePicker)]
+	public void DatePickerYearFormat_SetDateViaEntry_DisplaysFourDigitYear()
+	{
+		// Set a different date using the Entry field
+		App.WaitForElement("DateEntry");
+		App.ClearText("DateEntry");
+		App.EnterText("DateEntry", "1/1/2025");
+		App.Tap("SetDateButton");
+
+		// Verify DatePicker shows 4-digit year for the new date
+		var datePickerText = App.WaitForElement("MyDatePicker").GetText();
+		Assert.That(datePickerText, Does.Contain("2025"), "DatePicker should display 4-digit year for set date");
+
+		var displayLabelText = App.WaitForElement("DisplayLabel").GetText();
+		Assert.That(displayLabelText, Does.Contain("2025"), "Display label should show 4-digit year for set date");
+	}
+
+	[Test, Order(4)]
+	[Category(UITestCategories.DatePicker)]
+	public void DatePickerYearFormat_MultipleYearValues_AllShowFourDigits()
+	{
+		var testYears = new[] { "2020", "2023", "2026", "2030" };
+		var testDates = new[] { "1/1/2020", "2/2/2023", "3/3/2026", "4/4/2030" };
+
+		for (int i = 0; i < testDates.Length; i++)
+		{
+			// Set the test date
+			App.WaitForElement("DateEntry");
+			App.ClearText("DateEntry");
+			App.EnterText("DateEntry", testDates[i]);
+			App.Tap("SetDateButton");
+
+			// Verify 4-digit year display
+			var datePickerText = App.WaitForElement("MyDatePicker").GetText();
+			Assert.That(datePickerText, Does.Contain(testYears[i]),
+				$"DatePicker should display 4-digit year {testYears[i]} for date {testDates[i]}");
+		}
+	}
+
+	[Test, Order(5)]
+	[Category(UITestCategories.DatePicker)]
+	public void DatePickerYearFormat_iOS_ConsistentAfterMultipleInteractions()
+	{
+		// This test specifically validates the iOS year format fix through multiple interactions
+
+		// Set initial date
+		App.WaitForElement("DateEntry");
+		App.ClearText("DateEntry");
+		App.EnterText("DateEntry", "3/3/2024");
+		App.Tap("SetDateButton");
+
+		var initialText = App.WaitForElement("MyDatePicker").GetText();
+		Assert.That(initialText, Does.Contain("2024"), "Initial state should show 4-digit year");
+
+		// Perform multiple open/close cycles to test consistency
+		for (int i = 0; i < 3; i++)
+		{
+			// Open picker
+			App.Tap("MyDatePicker");
+			App.WaitForElement("Done");
+
+			// Close picker
+			App.Tap("Done");
+
+			// Verify format is still consistent
+			var currentText = App.WaitForElement("MyDatePicker").GetText();
+			Assert.That(currentText, Does.Contain("2024"),
+				$"After interaction #{i + 1}, should still show 4-digit year");
+			Assert.That(currentText, Is.EqualTo(initialText),
+				$"After interaction #{i + 1}, format should remain consistent");
+		}
+	}
+}
+#endif

src/Core/src/Platform/iOS/DatePickerExtensions.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Globalization;
+using System.Text.RegularExpressions;
 using Foundation;
 using UIKit;
 
@@ -79,23 +80,27 @@ public static void UpdateDate(this MauiDatePicker platformDatePicker, IDatePicke
 		{
 			platformDatePicker.Text = string.Empty;
 		}
-		else if (picker is not null && (string.IsNullOrWhiteSpace(format) || format.Equals("d", StringComparison.OrdinalIgnoreCase)))
+		else if (string.IsNullOrWhiteSpace(format) || format.Equals("d", StringComparison.OrdinalIgnoreCase))
 		{
 			NSDateFormatter dateFormatter = new NSDateFormatter
 			{
 				TimeZone = NSTimeZone.FromGMT(0)
 			};
 
+			// Use datePicker.Date (the source date) for formatting
+			// This ensures consistent formatting whether picker is initialized or not
+			var nsDate = datePicker.Date.Value.ToNSDate();
+
 			if (format.Equals("D", StringComparison.Ordinal) == true)
 			{
 				dateFormatter.DateStyle = NSDateFormatterStyle.Long;
-				var strDate = dateFormatter.StringFor(picker.Date);
+				var strDate = dateFormatter.StringFor(nsDate);
 				platformDatePicker.Text = strDate;
 			}
 			else
 			{
-				dateFormatter.DateStyle = NSDateFormatterStyle.Short;
-				var strDate = dateFormatter.StringFor(picker.Date);
+				dateFormatter.SetLocalizedDateFormatFromTemplate("yMd"); // Forces 4-digit year
+				var strDate = dateFormatter.StringFor(nsDate);
 				platformDatePicker.Text = strDate;
 			}
 		}