;; constants ;; Fixed second counts and calendar facts.
(define seconds-in-a-normal-year 31536000)
(define seconds-in-a-leap-year 31622400)
(define seconds-in-a-week 604800)
(define seconds-in-a-day 86400)
(define seconds-in-an-hour 3600)
(define base-year 1970)
(define month-length-normal-year (vector 31 28 31 30 31 30 31 31 30 31 30 31))
;; functions ;; The section of functions. ; Return whether y is a leap year
(define (leap-year y) (cond ((= (modulo y 400) 0) #t) ((= (modulo y 100) 0) #f) ((= (modulo y 4) 0) #t) (else #f))) ; Return a list of year and remainding seconds of the second ; counter n.
(define (years-and-seconds n) (cycle-years 0 base-year n)) ; Count years by increasing n and decreasing u until u is less than the seconds in a year. ; Both n and u are integeres representing a number of seconds. ; Returns a list of year and remaining seconds (less than a year).
(define (cycle-years n year u) (let ((year-length (if (leap-year year) seconds-in-a-leap-year seconds-in-a-normal-year))) (if (< u year-length) (list year u) (cycle-years (+ n year-length) (+ 1 year) (- u year-length))))) ; Return the number days, hours, minutes and seconds in second count n. ; n is less than the number of seconds in a year
(define (how-many-days-hours-minutes-seconds n) (let* ((days (quotient n seconds-in-a-day)) ; (n-rest-1 (modulo n seconds-in-a-day)) ; (hours (quotient n-rest-1 seconds-in-an-hour)) ; (n-rest-2 (modulo n-rest-1 seconds-in-an-hour)) ; (minutes (quotient n-rest-2 60)) ; (seconds (modulo n-rest-2 60))) ; (list days hours minutes seconds))) ; Return a list of day and moth given a number of days in year. ; Day-count is a number of days in a year.
(define (day-and-month day-count year) (day-and-month-help 0 1 year (+ 1 day-count)) )
(define (day-and-month-help n m y c) (if (<= c (days-in-month m y)) (list c m) (day-and-month-help (+ n (days-in-month m y)) (+ m 1) y (- c (days-in-month m y))))) ; Return the number of days in month and year
(define (days-in-month month year) (if (= month 2) (if (leap-year year) 29 28) (vector-ref month-length-normal-year (- month 1)))) ; Return a list of year month day hours minutes seconds from n, ; which represents the number of seconds elapsed since January 1, 1970.
(define (time-decode n) (let* ((year-seconds (years-and-seconds n)) ; (year (car year-seconds)) (days-hours-minutes-seconds ; (how-many-days-hours-minutes-seconds (cadr year-seconds))) (hours (second days-hours-minutes-seconds)) (minutes (third days-hours-minutes-seconds)) (seconds (fourth days-hours-minutes-seconds)) (day-month (day-and-month (first days-hours-minutes-seconds) year)) ; (day (first day-month)) (month (second day-month))) (list year month day hours minutes seconds)))