DouKaku?

(use srfi-13)
(use srfi-19)

(define (date-ex str delta)
  (let ((t (if (string-null? str)
               (current-time time-utc)
               (date->time-utc
                (string->date str "~Y-~m-~dT~H:~M:~S")))))
    (date->string
     (time-utc->date
      (add-duration t (make-time time-duration 0 delta)))
     "~5")))

(define (main args)
  (print (date-ex (cadr args) (string->number (caddr args)))))