[plt-scheme] SRFI-19 and week-numbers

Hi all,

I was attempting to find the start of the current week
and decided I'd try SRFI-19. The behaviour of its
date-week-number puzzles me.

The documentation reads:

     date-week-number date day-of-week-starting-week -> integer

     The ordinal week of the year which holds this date, ignoring a first
     partial week. 'Day-of-week-starting-week' is the integer
     corresponding to the day of the week which is to be considered the
     first day of the week (Sunday=0, Monday=1, etc.).

But see what happens:

(require (lib "19.ss" "srfi"))

; previous-date : date -> date
;   subtract 24 hours
(define (previous-date date)
   (time-monotonic->date
    (subtract-duration
     (date->time-monotonic date)
     (make-time 'time-duration
                0                ; nanoseconds
                (* 60 60 24))))) ; seconds in a 24 hours


(define (loop date n)
   (if (= n 0)
       '()
       (cons (list (date->string date)
                   ; monday is the first day of the week in Denmark
                   (date-week-number date 1))
             (loop (previous-date date) (- n 1)))))

 > (loop (current-date) 15)

(("Sun Sep 25 18:28:13+0200 2005" 38)
  ("Sat Sep 24 18:28:45+0200 2005" 37)
  ("Fri Sep 23 18:29:17+0200 2005" 37)
  ("Thu Sep 22 18:29:49+0200 2005" 37)
  ("Wed Sep 21 18:30:21+0200 2005" 37)
  ("Tue Sep 20 18:30:53+0200 2005" 37)
  ("Mon Sep 19 18:31:25+0200 2005" 37)
  ("Sun Sep 18 18:31:57+0200 2005" 37) ; Huh?
  ("Sat Sep 17 18:32:29+0200 2005" 36)
  ("Fri Sep 16 18:33:01+0200 2005" 36)
  ("Thu Sep 15 18:33:33+0200 2005" 36)
  ("Wed Sep 14 18:34:05+0200 2005" 36)
  ("Tue Sep 13 18:34:37+0200 2005" 36)
  ("Mon Sep 12 18:35:09+0200 2005" 36)
  ("Sun Sep 11 18:35:41+0200 2005" 36))

Using 2 = tuesday as the first day give
the expected:

(("Sun Sep 25 18:24:44+0200 2005" 37)
  ("Sat Sep 24 18:25:16+0200 2005" 37)
  ("Fri Sep 23 18:25:48+0200 2005" 37)
  ("Thu Sep 22 18:26:20+0200 2005" 37)
  ("Wed Sep 21 18:26:52+0200 2005" 37)
  ("Tue Sep 20 18:27:24+0200 2005" 37)
  ("Mon Sep 19 18:27:56+0200 2005" 37) ; !
  ("Sun Sep 18 18:28:28+0200 2005" 36)
  ("Sat Sep 17 18:29:00+0200 2005" 36)
  ("Fri Sep 16 18:29:32+0200 2005" 36)
  ("Thu Sep 15 18:30:04+0200 2005" 36)
  ("Wed Sep 14 18:30:36+0200 2005" 36)
  ("Tue Sep 13 18:31:08+0200 2005" 36)
  ("Mon Sep 12 18:31:40+0200 2005" 36)
  ("Sun Sep 11 18:32:12+0200 2005" 35))

Am I misunderstanding the documentation,
or have I found a bug?

-- 
Jens Axel Søgaard