anchor.diff:                              (:sec sec)
                               (:min minute)
                               (:hour hour)
  +                            (:hour12 (1+ (mod (1- hour) 12)))
                               (:day day)
                               (:weekday weekday)
                               (:month month)
  
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:   :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
    :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
  