Added UnixEpoch to DateTime and DateTimeOffset (#25235)

* Added UnixEpoch to DateTime and DateTimeOffset.

* Added tests

* Skip tests for netfx

* Move UnixEpoch tests to netcoreapp specific test files

Author: adriangodong

src/System.Runtime/ref/System.Runtime.cs

@@ -644,6 +644,7 @@ public CLSCompliantAttribute(bool isCompliant) { }
     {
         public static readonly System.DateTime MaxValue;
         public static readonly System.DateTime MinValue;
+        public static readonly System.DateTime UnixEpoch;
         public DateTime(int year, int month, int day) { throw null; }
         public DateTime(int year, int month, int day, System.Globalization.Calendar calendar) { throw null; }
         public DateTime(int year, int month, int day, int hour, int minute, int second) { throw null; }
@@ -772,6 +773,7 @@ public partial struct DateTimeOffset : System.IComparable, System.IComparable<Sy
     {
         public static readonly System.DateTimeOffset MaxValue;
         public static readonly System.DateTimeOffset MinValue;
+        public static readonly System.DateTimeOffset UnixEpoch;
         public DateTimeOffset(System.DateTime dateTime) { throw null; }
         public DateTimeOffset(System.DateTime dateTime, System.TimeSpan offset) { throw null; }
         public DateTimeOffset(int year, int month, int day, int hour, int minute, int second, int millisecond, System.Globalization.Calendar calendar, System.TimeSpan offset) { throw null; }

src/System.Runtime/tests/System/DateTimeOffsetTests.netcoreapp.cs

@@ -64,5 +64,11 @@ public static void TryFormat_ToString_EqualResults()
             Assert.Equal<char>(expectedString.ToCharArray(), actual.Slice(0, expectedString.Length).ToArray());
             Assert.Equal(0, actual[actual.Length - 1]);
         }
+
+        [Fact]
+        public static void UnixEpoch()
+        {
+            VerifyDateTimeOffset(DateTimeOffset.UnixEpoch, 1970, 1, 1, 0, 0, 0, 0, TimeSpan.Zero);
+        }
     }
 }

src/System.Runtime/tests/System/DateTimeTests.netcoreapp.cs

@@ -82,5 +82,11 @@ public static void ParseExact_Span_InvalidInputs_Fail(string input, string forma
             Assert.False(DateTime.TryParseExact(input.AsReadOnlySpan(), format, culture, style, out DateTime result));
             Assert.False(DateTime.TryParseExact(input.AsReadOnlySpan(), new[] { format }, culture, style, out result));
         }
+
+        [Fact]
+        public static void UnixEpoch()
+        {
+            VerifyDateTime(DateTime.UnixEpoch, 1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
+        }
     }
 }