anchor.diff:    :SHORT-WEEKDAY     short form of weekday (e.g. Sun, Mon)
     :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
  +  :HOUR12            *hour on a 12-hour clock
     :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
  
positive.code:                (princ (aref +short-day-names+ weekday) result))
                ((eql fmt :timezone)
                 (princ abbrev result))
                ((eql fmt :hour12)
                 (princ (1+ (mod (1- hour) 12)) result))
                ((eql fmt :ampm)
                 (princ (if (< hour 12) "am" "pm") result))
                ((eql fmt :ordinal-day)
  
negative.code:                             (:sec sec)
                              (:min minute)
                              (:hour hour)
                              (:hour12 (1+ (mod (1- hour) 12)))
                              (:day day)
                              (:weekday weekday)
                              (:month month)
  