Add polyfill for DateTimeOffset.UnixEpoch (#33)

Author: Tyrrrz

PolyShim/NetCore21/DateTimeOffset.cs

@@ -0,0 +1,21 @@
+// The following comment is required to instruct analyzers to skip this file
+// <auto-generated/>
+
+#if (NETCOREAPP && !NETCOREAPP2_1_OR_GREATER) || (NETFRAMEWORK) || (NETSTANDARD && !NETSTANDARD2_1_OR_GREATER)
+#nullable enable
+// ReSharper disable RedundantUsingDirective
+// ReSharper disable CheckNamespace
+// ReSharper disable InconsistentNaming
+// ReSharper disable PartialTypeWithSinglePart
+
+using System;
+
+internal static partial class PolyfillExtensions
+{
+    extension(DateTimeOffset)
+    {
+        // https://learn.microsoft.com/en-us/dotnet/api/system.datetimeoffset.unixepoch?view=net-9.0
+        public static DateTimeOffset UnixEpoch => new(1970, 1, 1, 0, 0, 0, TimeSpan.Zero);
+    }
+}
+#endif