Date Calculation

These snippets contains three small function for calculating the first week-day in a month, whether a year is a leap year, and the number of days in a month.

  ;; ,open srfi-19

  ;; return the first weekday in a month as a number
  ;; (0 for sunday, 1 for monday, ...)

  (define (first-week-day month year)
    (date-week-day (make-date 0 0 0 0 1 month year 0)))

  (define days-per-month-vector
    (vector 31 28 31 30 31 30 31 31 30 31 30 31))

  (define (leap-year? year)
    (cond
     ((zero? (modulo year 400)) #t)
     ((zero? (modulo year 100)) #f)
     ((zero? (modulo year 4)) #t)
     (else #f)))

  (define (days-in-month month year)
     (if (= month 2)
         (if (leap-year? year) 
           29
           28)
         (vector-ref days-per-month-vector 
                   (- month 1))))

-Eric

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