Mark StartTime as experimental

src/Libraries/Microsoft.Extensions.TimeProvider.Testing/ExperimentalAttribute.cs

@@ -0,0 +1,53 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Diagnostics.CodeAnalysis;
+
+/// <summary>
+/// Indicates that an API element is experimental and subject to change without notice.
+/// </summary>
+[ExcludeFromCodeCoverage]
+[AttributeUsage(
+    AttributeTargets.Class |
+    AttributeTargets.Struct |
+    AttributeTargets.Enum |
+    AttributeTargets.Interface |
+    AttributeTargets.Delegate |
+    AttributeTargets.Method |
+    AttributeTargets.Constructor |
+    AttributeTargets.Property |
+    AttributeTargets.Field |
+    AttributeTargets.Event |
+    AttributeTargets.Assembly)]
+internal sealed class ExperimentalAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ExperimentalAttribute"/> class.
+    /// </summary>
+    public ExperimentalAttribute()
+    {
+        // Intentionally left empty.
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ExperimentalAttribute"/> class.
+    /// </summary>
+    /// <param name="message">Human readable explanation for marking experimental API.</param>
+    public ExperimentalAttribute(string message)
+    {
+#pragma warning disable R9A014 // Use the 'Microsoft.Extensions.Diagnostics.Throws' class instead of explicitly throwing exception for improved performance
+#pragma warning disable R9A039 // Remove superfluous null check when compiling in a nullable context
+#pragma warning disable R9A060 // Consider removing unnecessary null coalescing (??) since the left-hand value is statically known not to be null
+#pragma warning disable SA1101 // Prefix local calls with this
+        Message = message ?? throw new ArgumentNullException(nameof(message));
+#pragma warning restore SA1101 // Prefix local calls with this
+#pragma warning restore R9A060 // Consider removing unnecessary null coalescing (??) since the left-hand value is statically known not to be null
+#pragma warning restore R9A039 // Remove superfluous null check when compiling in a nullable context
+#pragma warning restore R9A014 // Use the 'Microsoft.Extensions.Diagnostics.Throws' class instead of explicitly throwing exception for improved performance
+    }
+
+    /// <summary>
+    /// Gets a human readable explanation for marking API as experimental.
+    /// </summary>
+    public string? Message { get; }
+}

src/Libraries/Microsoft.Extensions.TimeProvider.Testing/FakeTimeProvider.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Threading;
 using Microsoft.Shared.Diagnostics;
@@ -23,6 +24,7 @@ public class FakeTimeProvider : TimeProvider
     /// <summary>
     /// Gets the time at which the instance of the <see cref="FakeTimeProvider"/> was initialized.
     /// </summary>
+    [Experimental]
     public DateTimeOffset StartTime { get; } = new DateTimeOffset(2000, 1, 1, 0, 0, 0, 0, TimeSpan.Zero);
 
     /// <summary>