diff --git a/core/sys/clock.h b/core/sys/clock.h
index f06e8a436..1fdcb0efa 100644
--- a/core/sys/clock.h
+++ b/core/sys/clock.h
@@ -53,14 +53,23 @@
  *
  * Author: Adam Dunkels <adam@sics.se>
  *
- * $Id: clock.h,v 1.6 2008/07/10 17:09:32 adamdunkels Exp $
+ * $Id: clock.h,v 1.7 2008/08/15 19:16:09 adamdunkels Exp $
  */
 #ifndef __CLOCK_H__
 #define __CLOCK_H__
 
 #include "contiki-conf.h"
 
-#define CLOCK_LT(a,b)     ((signed short)((a)-(b)) < 0)
+/**
+ * Check if a clock time value is less than another clock time value.
+ *
+ * This macro checks if a clock time value is less than another clock
+ * time value. This macro is needed to correctly handle wrap-around of
+ * clock time values.
+ *
+ */
+#define CLOCK_LT(a, b) ((clock_time_t)((a) - (b)) < \
+                       ((clock_time_t)(~((clock_time_t)0)) >> 1))
 
 /**
  * Initialize the clock library.
diff --git a/core/sys/timer.c b/core/sys/timer.c
index 2719834db..064af4b74 100644
--- a/core/sys/timer.c
+++ b/core/sys/timer.c
@@ -12,37 +12,37 @@
 
 /*
  * Copyright (c) 2004, Swedish Institute of Computer Science.
- * All rights reserved. 
+ * All rights reserved.
  *
- * Redistribution and use in source and binary forms, with or without 
- * modification, are permitted provided that the following conditions 
- * are met: 
- * 1. Redistributions of source code must retain the above copyright 
- *    notice, this list of conditions and the following disclaimer. 
- * 2. Redistributions in binary form must reproduce the above copyright 
- *    notice, this list of conditions and the following disclaimer in the 
- *    documentation and/or other materials provided with the distribution. 
- * 3. Neither the name of the Institute nor the names of its contributors 
- *    may be used to endorse or promote products derived from this software 
- *    without specific prior written permission. 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the Institute nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
  *
- * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND 
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE 
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 
- * SUCH DAMAGE. 
+ * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
  *
  * This file is part of the Contiki operating system.
- * 
+ *
  * Author: Adam Dunkels <adam@sics.se>
  *
- * $Id: timer.c,v 1.2 2007/05/27 11:11:28 oliverschmidt Exp $
+ * $Id: timer.c,v 1.3 2008/08/15 19:16:09 adamdunkels Exp $
  */
 
 #include "contiki-conf.h"
@@ -121,7 +121,7 @@ timer_restart(struct timer *t)
 int
 timer_expired(struct timer *t)
 {
-  return (clock_time_t)(clock_time() - t->start) >= (clock_time_t)t->interval;
+  return CLOCK_LT(clock_time(), t->start + t->interval);
 }
 /*---------------------------------------------------------------------------*/