Hello. I was reading cal(1)'s source code.

The URL in the comment of cal(1)'s source code is outdated. That URL
pointed the documentation about an algorithm that computes the week
number of a given day. The original documentation says the new
documentation is at the URL below.

     https://www.tondering.dk/claus/cal/week.php

There is a diff below. I think it could be just deleted not updated
because external documentations can be obsleted anytime.

Best regards,
Biarder

Index: cal.c
===================================================================
RCS file: /cvs/src/usr.bin/cal/cal.c,v
diff -u -p -r1.33 cal.c
--- cal.c	26 Jun 2026 12:34:45 -0000	1.33
+++ cal.c	28 Jun 2026 20:11:44 -0000
@@ -265,7 +265,7 @@ week(int day, int month, int year)
 int
 isoweek(int day, int month, int year)
 {
-	/* http://www.tondering.dk/claus/cal/node8.html */
+	/* https://www.tondering.dk/claus/cal/week.php */
 	int a, b, c, s, e, f, g, d, n;
 
 	a = month <= 2 ? year - 1 : year;