diff --git a/ext/date/lib/interval.c b/ext/date/lib/interval.c
index c8200a48c34..af150fc8023 100644
--- a/ext/date/lib/interval.c
+++ b/ext/date/lib/interval.c
@@ -19,6 +19,7 @@
 /* $Id$ */
 
 #include "timelib.h"
+#include <math.h>
 
 timelib_rel_time *timelib_diff(timelib_time *one, timelib_time *two)
 {