diff --git a/Core/EwsUtilities.cs b/Core/EwsUtilities.cs
index 2076f36b..dd9dcbbd 100644
--- a/Core/EwsUtilities.cs
+++ b/Core/EwsUtilities.cs
@@ -406,7 +406,7 @@ private static void WriteTraceStartElement(
             writer.WriteStartElement("Trace");
             writer.WriteAttributeString("Tag", traceTag);
             writer.WriteAttributeString("Tid", Thread.CurrentThread.ManagedThreadId.ToString());
-            writer.WriteAttributeString("Time", DateTime.UtcNow.ToString("u", DateTimeFormatInfo.InvariantInfo));
+            writer.WriteAttributeString("Time", DateTime.UtcNow.ToString("O", DateTimeFormatInfo.InvariantInfo));
 
             if (includeVersion)
             {
@@ -1603,4 +1603,4 @@ internal static void ForEach<T>(this IEnumerable<T> collection, Action<T> action
         }
         #endregion
     }
-}
\ No newline at end of file
+}