diff --git a/src/Microsoft.AspNet.Server.Kestrel/Http/DateHeaderValueManager.cs b/src/Microsoft.AspNet.Server.Kestrel/Http/DateHeaderValueManager.cs
index 04dc75f464..c471067eca 100644
--- a/src/Microsoft.AspNet.Server.Kestrel/Http/DateHeaderValueManager.cs
+++ b/src/Microsoft.AspNet.Server.Kestrel/Http/DateHeaderValueManager.cs
@@ -59,7 +59,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
             // string used here.
             // The null-coalesce here is to protect against returning null after Dispose() is called, at which
             // point _dateValue will be null forever after.
-            return _dateValue ?? _systemClock.UtcNow.ToString("r");
+            return _dateValue ?? _systemClock.UtcNow.ToString(Constants.RFC1123DateFormat);
         }
 
         /// <summary>
@@ -69,11 +69,8 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
         {
             lock (_timerLocker)
             {
-                if (_dateValueTimer != null)
-                {
-                    DisposeTimer();
-                }
-
+                DisposeTimer();
+                
                 _isDisposed = true;
             }
         }
@@ -94,7 +91,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
                         // Immediately assign the date value and start the timer again. We assign the value immediately
                         // here as the timer won't fire until the timer interval has passed and we want a value assigned
                         // inline now to serve requests that occur in the meantime.
-                        _dateValue = _systemClock.UtcNow.ToString("r");
+                        _dateValue = _systemClock.UtcNow.ToString(Constants.RFC1123DateFormat);
                         _dateValueTimer = new Timer(UpdateDateValue, state: null, dueTime: _timerInterval, period: _timerInterval);
                     }
                 }
@@ -107,7 +104,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
             var now = _systemClock.UtcNow;
 
             // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.18 for required format of Date header
-            _dateValue = now.ToString("r");
+            _dateValue = now.ToString(Constants.RFC1123DateFormat);
 
             if (_hadRequestsSinceLastTimerTick)
             {
@@ -137,9 +134,12 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
         private void DisposeTimer()
         {
-            _dateValueTimer.Dispose();
-            _dateValueTimer = null;
-            _dateValue = null;
+            if (_dateValueTimer != null)
+            {
+                _dateValueTimer.Dispose();
+                _dateValueTimer = null;
+                _dateValue = null;
+            }
         }
     }
 }
diff --git a/src/Microsoft.AspNet.Server.Kestrel/Infrastructure/Constants.cs b/src/Microsoft.AspNet.Server.Kestrel/Infrastructure/Constants.cs
index 0f4b72424e..9b72df3361 100644
--- a/src/Microsoft.AspNet.Server.Kestrel/Infrastructure/Constants.cs
+++ b/src/Microsoft.AspNet.Server.Kestrel/Infrastructure/Constants.cs
@@ -14,5 +14,11 @@ namespace Microsoft.AspNet.Server.Kestrel.Infrastructure
         /// Prefix of host name used to specify Unix sockets in the configuration.
         /// </summary>
         public const string UnixPipeHostPrefix = "unix:/";
+
+        /// <summary>
+        /// DateTime format string for RFC1123. See  https://msdn.microsoft.com/en-us/library/az4se3k1(v=vs.110).aspx#RFC1123
+        /// for info on the format.
+        /// </summary>
+        public const string RFC1123DateFormat = "r";
     }
 }
diff --git a/test/Microsoft.AspNet.Server.KestrelTests/DateHeaderValueManagerTests.cs b/test/Microsoft.AspNet.Server.KestrelTests/DateHeaderValueManagerTests.cs
index 71c294caae..ca4fd8ff01 100644
--- a/test/Microsoft.AspNet.Server.KestrelTests/DateHeaderValueManagerTests.cs
+++ b/test/Microsoft.AspNet.Server.KestrelTests/DateHeaderValueManagerTests.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Server.Kestrel.Http;
+using Microsoft.AspNet.Server.Kestrel.Infrastructure;
 using Microsoft.AspNet.Server.KestrelTests.TestHelpers;
 using Xunit;
 
@@ -33,7 +34,7 @@ namespace Microsoft.AspNet.Server.KestrelTests
                 dateHeaderValueManager.Dispose();
             }
 
-            Assert.Equal(now.ToString("r"), result);
+            Assert.Equal(now.ToString(Constants.RFC1123DateFormat), result);
         }
 
         [Fact]
@@ -62,8 +63,8 @@ namespace Microsoft.AspNet.Server.KestrelTests
                 dateHeaderValueManager.Dispose();
             }
 
-            Assert.Equal(now.ToString("r"), result1);
-            Assert.Equal(now.ToString("r"), result2);
+            Assert.Equal(now.ToString(Constants.RFC1123DateFormat), result1);
+            Assert.Equal(now.ToString(Constants.RFC1123DateFormat), result2);
             Assert.Equal(1, systemClock.UtcNowCalled);
         }
 
@@ -95,8 +96,8 @@ namespace Microsoft.AspNet.Server.KestrelTests
                 dateHeaderValueManager.Dispose();
             }
 
-            Assert.Equal(now.ToString("r"), result1);
-            Assert.Equal(future.ToString("r"), result2);
+            Assert.Equal(now.ToString(Constants.RFC1123DateFormat), result1);
+            Assert.Equal(future.ToString(Constants.RFC1123DateFormat), result2);
             Assert.True(systemClock.UtcNowCalled >= 2);
         }
 
@@ -118,8 +119,8 @@ namespace Microsoft.AspNet.Server.KestrelTests
             systemClock.UtcNow = future;
             var result2 = dateHeaderValueManager.GetDateHeaderValue();
             
-            Assert.Equal(now.ToString("r"), result1);
-            Assert.Equal(future.ToString("r"), result2);
+            Assert.Equal(now.ToString(Constants.RFC1123DateFormat), result1);
+            Assert.Equal(future.ToString(Constants.RFC1123DateFormat), result2);
         }
     }
 }