X-Git-Url: http://git.kpe.io/?p=kmrcl.git;a=blobdiff_plain;f=datetime.lisp;h=8357da030b95b5e028d80711e072e6fb83cfdc30;hp=0346661cff309be2d75acf6480ccced646ab91bd;hb=22765cd49d01672b58873f98554c3b5f3069663f;hpb=bcccfec7f76c7ab04b1228255492086dc31179b0

diff --git a/datetime.lisp b/datetime.lisp
index 0346661..8357da0 100644
--- a/datetime.lisp
+++ b/datetime.lisp
@@ -32,7 +32,7 @@
 		   "July" "August" "September" "October" "November"
 		   "December")
 		 (1- mn))
-	    (format nil "~A" dy) 
+	    (format nil "~A" dy)
             (format nil "~A" yr)
 	    (format nil "~2,'0D:~2,'0D:~2,'0D" hr min sec))))
 
@@ -108,6 +108,20 @@
               mincol colinc minpad padchar
               (subseq monthstring 0 truncate)))))
 
+(defconstant* +zellers-adj+ #(0 3 2 5 0 3 5 1 4 6 2 4))
+
+(defun day-of-week (year month day)
+  "Day of week calculation using Zeller's Congruence.
+Input: The year y, month m (1 ≤ m ≤ 12) and day d (1 ≤ d ≤ 31).
+Output: n - the day of the week (Sunday = 0, Saturday = 6)."
+
+  (when (< month 3)
+    (decf year))
+  (mod
+   (+ year (floor year 4) (- (floor year 100)) (floor year 400)
+      (aref +zellers-adj+ (1- month)) day)
+   7))
+
 ;;;; Daylight Saving Time calculations
 
 ;; Daylight Saving Time begins for most of the United States at 2