Fall back to loading .NET 8 date strings

Fixes #25930

src/Essentials/src/Preferences/Preferences.uwp.cs

@@ -212,12 +212,16 @@ public T Get<T>(string key, T defaultValue, string sharedName = null)
 				{
 					if (defaultValue is DateTime dt)
 					{
-						long tempValue = (long)Convert.ChangeType(value, typeof(long), CultureInfo.InvariantCulture);
-						return (T)(object)DateTime.FromBinary(tempValue);
+						// long for the .NET 9+ format
+						if (long.TryParse(value, CultureInfo.InvariantCulture, out var longValue))
+							return (T)(object)DateTime.FromBinary(longValue);
+						// DateTime string for the .NET 8 format
+						if (DateTime.TryParse(value, CultureInfo.InvariantCulture, out var datetimeValue))
+							return (T)(object)datetimeValue;
 					}
 					else if (defaultValue is DateTimeOffset dto)
 					{
-						if (DateTimeOffset.TryParse((string)value, out var dateTimeOffset))
+						if (DateTimeOffset.TryParse((string)value, CultureInfo.InvariantCulture, out var dateTimeOffset))
 						{
 							return (T)(object)dateTimeOffset;
 						}

src/Essentials/test/DeviceTests/Tests/Preferences_Tests.cs

@@ -246,6 +246,27 @@ public void DateTimePreservesKind(string sharedName, DateTimeKind kind)
 		public void FailsWithUnsupportedType() =>
 			Assert.Throws<NotSupportedException>(() => Preferences.Default.Set("anything", new int[] { 1 }));
 
+#if WINDOWS
+		[Fact]
+		public void DateTime_Supports_Reading_String_Compat()
+		{
+			// This is a special test where when unpackaged on windows in .NET 8
+			// dates were stored as ToString but in .NET 6 they are stored as ToBinary.
+			// This test ensures that the compat layer is working correctly
+			// and that the date is read correctly regardless of the storage format.
+			// This test is only valid on windows unpackaged.
+
+			if (ApplicationModel.AppInfoUtils.IsPackagedApp)
+				return;
+
+			Preferences.Default.Set("datetime_compat", testDateTime.ToString(), null);
+
+			var get = Preferences.Default.Get("datetime_compat", DateTime.MinValue, null);
+
+			Assert.Equal(testDateTime, get);
+		}
+#endif
+
 		[Theory]
 		[InlineData("datetime1", null)]
 		[InlineData("datetime1", sharedNameTestData)]