fix(#51740): Adding comma as valid time fraction delimiter.

Author: Maksim Golev

src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs

@@ -14,6 +14,7 @@ internal static class DateTimeParse
         internal const int MaxDateTimeNumberDigits = 8;
 
         internal const char TimeDelimiter = ':';
+        internal const char TimeFractionDelimiterComma = ',';
         internal const char TimeFractionDelimiterDot = '.';
 
         internal static DateTime ParseExact(ReadOnlySpan<char> s, ReadOnlySpan<char> format, DateTimeFormatInfo dtfi, DateTimeStyles style)
@@ -645,7 +646,8 @@ private static bool Lex(DS dps, ref __DTString str, scoped ref DateTimeToken dto
                         if (str.Index < str.Length - 1)
                         {
                             char nextCh = str.Value[str.Index];
-                            if (nextCh == TimeFractionDelimiterDot)
+                            if ((nextCh == TimeFractionDelimiterDot)
+                                || (nextCh == TimeFractionDelimiterComma))
                             {
                                 // While ParseFraction can fail, it just means that there were no digits after
                                 // the dot. In this case ParseFraction just removes the dot. This is actually
@@ -2984,7 +2986,8 @@ private static bool ParseISO8601(scoped ref DateTimeRawInfo raw, ref __DTString
                     result.SetBadDateTimeFailure();
                     return false;
                 }
-                if (str.Match(TimeFractionDelimiterDot))
+                if ((str.Match(TimeFractionDelimiterDot))
+                    || (str.Match(TimeFractionDelimiterComma)))
                 {
                     if (!ParseFraction(ref str, out partSecond))
                     {

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

@@ -856,14 +856,18 @@ public static void Compare(DateTimeOffset dateTimeOffset1, DateTimeOffset dateTi
             }
         }
 
-        [Fact]
-        public static void Parse_String()
+        public static IEnumerable<object[]> Parse_TestData()
         {
-            DateTimeOffset expected = DateTimeOffset.MaxValue;
-            string expectedString = expected.ToString();
+            yield return new object[] { "2021-04-23T13:04:17,307642270+02:00", new DateTimeOffset(637547798573076423, new TimeSpan(2, 0, 0)) };
+            yield return new object[] { DateTimeOffset.MaxValue.ToString("O"), DateTimeOffset.MaxValue };
+        }
 
-            DateTimeOffset result = DateTimeOffset.Parse(expectedString);
-            Assert.Equal(expectedString, result.ToString());
+        [Theory]
+        [MemberData(nameof(Parse_TestData))]
+        public static void Parse_String(string valueForParse, DateTimeOffset expectedValue)
+        {
+            DateTimeOffset actualValue = DateTimeOffset.Parse(valueForParse);
+            Assert.Equal(expectedValue, actualValue);
         }
 
         [Fact]