diff --git a/src/not-used.c b/src/not-used.c
index 8621ac2..027d459 100644
--- a/src/not-used.c
+++ b/src/not-used.c
@@ -1,3 +1,11 @@
+/** Tyler Durden: calculates a day number according to the Gregorian calendar. */
+function g(y,m,d)
+m = (m + 9) % 12
+y = y - m/10
+return 365*y + y/4 - y/100 + y/400 + (m*306 + 5)/10 + ( d - 1 )
+
+Difference between two dates = g(y2,m2,d2) - g(y1,m1,d1)
+
 /** Tomohiko Sakamoto comp.lang.c 1993-04-10. */
 static unsigned weekday(union date32 d) {
 	d.year -= d.month < 3;