guile-zenity/examples/birthday.scm

(use-modules (zenity)
             (srfi srfi-19))

(define (seconds->years s)
  (floor (/ s 60 60 24 356)))

(let* ((birthday (or (zenity-calendar "What is your date of birth?")
                     (exit)))
       (now (current-date))
       (birthday* (make-date 0 0 0 0
                             (cdr (assoc 'day birthday))
                             (cdr (assoc 'month birthday))
                             (cdr (assoc 'year birthday))
                             (date-zone-offset now))))
  (when (= (date-year-day now)
           (date-year-day birthday*))
    (display "Happy birthday!")
    (newline))
  (let* ((age-sec (time-second (time-difference (date->time-utc now)
                                                (date->time-utc birthday*))))
         (age (seconds->years age-sec)))
    (display (list "You are" age "years old"))
    (newline)))