Added custom DateTimeFormatter

src/Microsoft.Net.Http.Headers/ContentDispositionHeaderValue.cs

@@ -321,7 +321,7 @@ private void SetDate(string parameter, DateTimeOffset? date)
             else
             {
                 // Must always be quoted
-                var dateString = string.Format(CultureInfo.InvariantCulture, "\"{0}\"", HttpRuleParser.DateToString(date.Value));
+                var dateString = HeaderUtilities.FormatDate(date.Value, quoted: true);
                 if (dateParameter != null)
                 {
                     dateParameter.Value = dateString;

src/Microsoft.Net.Http.Headers/DateTimeFormatter.cs

@@ -0,0 +1,100 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Globalization;
+using System.Runtime.CompilerServices;
+using Microsoft.Extensions.Primitives;
+
+namespace Microsoft.Net.Http.Headers
+{
+    internal static class DateTimeFormatter
+    {
+        private static readonly DateTimeFormatInfo FormatInfo = CultureInfo.InvariantCulture.DateTimeFormat;
+
+        private static readonly string[] MonthNames = FormatInfo.AbbreviatedMonthNames;
+        private static readonly string[] DayNames = FormatInfo.AbbreviatedDayNames;
+
+        private static readonly int Rfc1123DateLength = "ddd, dd MMM yyyy HH:mm:ss GMT".Length;
+        private static readonly int QuotedRfc1123DateLength = Rfc1123DateLength + 2;
+
+        // ASCII numbers are in the range 48 - 57.
+        private const int AsciiNumberOffset = 0x30;
+
+        private const string Gmt = "GMT";
+        private const char Comma = ',';
+        private const char Space = ' ';
+        private const char Colon = ':';
+        private const char Quote = '"';
+
+        public static string ToRfc1123String(this DateTimeOffset dateTime)
+        {
+            return ToRfc1123String(dateTime, false);
+        }
+
+        public static string ToRfc1123String(this DateTimeOffset dateTime, bool quoted)
+        {
+            var universal = dateTime.UtcDateTime;
+
+            var length = quoted ? QuotedRfc1123DateLength : Rfc1123DateLength;
+            var target = new InplaceStringBuilder(length);
+
+            if (quoted)
+            {
+                target.Append(Quote);
+            }
+
+            target.Append(DayNames[(int)universal.DayOfWeek]);
+            target.Append(Comma);
+            target.Append(Space);
+            AppendNumber(ref target, universal.Day);
+            target.Append(Space);
+            target.Append(MonthNames[universal.Month - 1]);
+            target.Append(Space);
+            AppendYear(ref target, universal.Year);
+            target.Append(Space);
+            AppendTimeOfDay(ref target, universal.TimeOfDay);
+            target.Append(Space);
+            target.Append(Gmt);
+
+            if (quoted)
+            {
+                target.Append(Quote);
+            }
+
+            return target.ToString();
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static void AppendYear(ref InplaceStringBuilder target, int year)
+        {
+            target.Append(GetAsciiChar(year / 1000));
+            target.Append(GetAsciiChar(year % 1000 / 100));
+            target.Append(GetAsciiChar(year % 100 / 10));
+            target.Append(GetAsciiChar(year % 10));
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static void AppendTimeOfDay(ref InplaceStringBuilder target, TimeSpan timeOfDay)
+        {
+            AppendNumber(ref target, timeOfDay.Hours);
+            target.Append(Colon);
+            AppendNumber(ref target, timeOfDay.Minutes);
+            target.Append(Colon);
+            AppendNumber(ref target, timeOfDay.Seconds);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static void AppendNumber(ref InplaceStringBuilder target, int number)
+        {
+            target.Append(GetAsciiChar(number / 10));
+            target.Append(GetAsciiChar(number % 10));
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static char GetAsciiChar(int value)
+        {
+            return (char)(AsciiNumberOffset + value);
+        }
+    }
+}

src/Microsoft.Net.Http.Headers/HeaderUtilities.cs

@@ -220,7 +220,12 @@ public static bool TryParseDate(string input, out DateTimeOffset result)
 
         public static string FormatDate(DateTimeOffset dateTime)
         {
-            return HttpRuleParser.DateToString(dateTime);
+            return FormatDate(dateTime, false);
+        }
+
+        public static string FormatDate(DateTimeOffset dateTime, bool quoted)
+        {
+            return dateTime.ToRfc1123String(quoted);
         }
 
         public static string RemoveQuotes(string input)

src/Microsoft.Net.Http.Headers/HttpRuleParser.cs

@@ -233,12 +233,6 @@ internal static HttpParseResult GetQuotedPairLength(string input, int startIndex
             return HttpParseResult.Parsed;
         }
 
-        internal static string DateToString(DateTimeOffset dateTime)
-        {
-            // Format according to RFC1123; 'r' uses invariant info (DateTimeFormatInfo.InvariantInfo)
-            return dateTime.ToUniversalTime().ToString("r", CultureInfo.InvariantCulture);
-        }
-
         internal static bool TryStringToDate(string input, out DateTimeOffset result)
         {
             // Try the various date formats in the order listed above.

src/Microsoft.Net.Http.Headers/RangeConditionHeaderValue.cs

@@ -53,7 +53,7 @@ public override string ToString()
         {
             if (_entityTag == null)
             {
-                return HttpRuleParser.DateToString(_lastModified.Value);
+                return HeaderUtilities.FormatDate(_lastModified.Value);
             }
             return _entityTag.ToString();
         }

src/Microsoft.Net.Http.Headers/project.json

@@ -24,6 +24,7 @@
   "frameworks": {
     "netstandard1.1": {
       "dependencies": {
+        "Microsoft.Extensions.Primitives": "1.1.0-*",
         "System.Buffers": "4.3.0-*",
         "System.Diagnostics.Contracts": "4.3.0-*"
       }

test/Microsoft.Net.Http.Headers.Tests/HeaderUtilitiesTest.cs

@@ -0,0 +1,48 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Xunit;
+
+namespace Microsoft.Net.Http.Headers
+{
+    public static class HeaderUtilitiesTest
+    {
+        private const string Rfc1123Format = "r";
+
+        [Theory]
+        [MemberData(nameof(TestValues))]
+        public static void ReturnsSameResultAsRfc1123String(DateTimeOffset dateTime, bool quoted)
+        {
+            var formatted = dateTime.ToString(Rfc1123Format);
+            var expected = quoted ? $"\"{formatted}\"" : formatted;
+            var actual = HeaderUtilities.FormatDate(dateTime, quoted);
+
+            Assert.Equal(expected, actual);
+        }
+
+        public static TheoryData<DateTimeOffset, bool> TestValues
+        {
+            get
+            {
+                var data = new TheoryData<DateTimeOffset, bool>();
+
+                var now = DateTimeOffset.Now;
+
+                foreach (var quoted in new[] { true, false })
+                {
+                    for (var i = 0; i < 60; i++)
+                    {
+                        data.Add(now.AddSeconds(i), quoted);
+                        data.Add(now.AddMinutes(i), quoted);
+                        data.Add(now.AddDays(i), quoted);
+                        data.Add(now.AddMonths(i), quoted);
+                        data.Add(now.AddYears(i), quoted);
+                    }
+                }
+
+                return data;
+            }
+        }
+    }
+}