private void AssertTimestamp(Timestamp timestamp, SoapContext context)
        {
            // TODO: add clock skew options
            var now = Clock.UtcNow.UtcDateTime;

            if (timestamp.Created.AddMinutes(-5).ToUniversalTime() > now || timestamp.Expires.ToUniversalTime() < now)
            {
                throw context.CreateMessageExpiredFault();
            }
        }