CL-USER> (defstruct (date (:print-function
                           (lambda (p s k)
                             (declare (ignore k))
                             (format s "~4,'0d-~2,'0d-~2,'0d"
                                     (date-year  p)
                                     (date-month p)
                                     (date-day   p)))))
           "For example, (1900-01-01) is a date."
           (year  1900     :type number)
           (month 01       :type number)
           (day   01       :type number))
DATE

CL-USER> (make-date :year 2020 :month 12 :day 14)
2020-12-14

CL-USER> (prin1-to-string (make-date :year 2020 :month 12 :day 14))
"2020-12-14"