anchor.diff:    :AMPM              am/pm marker in lowercase
     :GMT-OFFSET        the gmt-offset of the time, in +00:00 form
     :GMT-OFFSET-OR-Z   like :GMT-OFFSET, but is Z when UTC
  +  :GMT-OFFSET-HHMM   like :GMT-OFFSET, but in +0000 form
     :TIMEZONE          timezone abbrevation for the time
   
   Elements marked by * can be placed in a list in the form: \(:keyword padding &optional \(padchar #\0))
  
positive.code: (defparameter +rfc-1123-format+
    ;; Sun, 06 Nov 1994 08:49:37 GMT
    '(:short-weekday ", " (:day 2) #\space :short-month #\space (:year 4) #\space
      (:hour 2) #\: (:min 2) #\: (:sec 2) #\space :timezone)
    "Please note that you should use the +GMT-ZONE+ timezone to format a proper RFC 1123 timestring. See the RFC for the details about the possible values of the timezone field.")
  
  (defparameter +iso-week-date-format+
    ;; 2009-W53-5
  
negative.code:       (write-string result (if (eq t destination) *standard-output* destination)))
      result))
  
  (defun format-rfc1123-timestring (destination timestamp &key
                                    (timezone *default-timezone*))
    (format-timestring destination timestamp
                       :format +rfc-1123-format+
                       :timezone timezone))
  
  (defun to-rfc1123-timestring (timestamp)
    (format-rfc1123-timestring nil timestamp))
  