anchor.diff:    :ISO-WEEK-DAY      *ISO compatible weekday number (monday=1, sunday=7)
     :LONG-WEEKDAY      long form of weekday (e.g. Sunday, Monday)
     :SHORT-WEEKDAY     short form of weekday (e.g. Sun, Mon)
  +  :MINIMAL-WEEKDAY   minimal form of weekday (e.g. Su, Mo)
     :LONG-MONTH        long form of month (e.g. January, February)
     :SHORT-MONTH       short form of month (e.g. Jan, Feb)
     :HOUR12            *hour on a 12-hour clock
  
positive.code:                (princ (aref +day-names+ weekday) result))
                ((eql fmt :short-weekday)
                 (princ (aref +short-day-names+ weekday) result))
                ((eql fmt :timezone)
                 (princ abbrev result))
                ((eql fmt :ampm)
  
negative.code:   #(:sunday :monday :tuesday :wednesday :thursday :friday :saturday))
  (defparameter +short-day-names+
    #("Sun" "Mon" "Tue" "Wed" "Thu" "Fri" "Sat"))
  (defparameter +minimal-day-names+
    #("Su" "Mo" "Tu" "We" "Th" "Fr" "Sa"))
  
  (eval-when (:compile-toplevel :load-toplevel :execute)
    (defconstant +months-per-year+ 12)
  