diff --git a/sys/kern/kern_time.c b/sys/kern/kern_time.c
index 9479401719a0..e25ddd72ac64 100644
--- a/sys/kern/kern_time.c
+++ b/sys/kern/kern_time.c
@@ -851,10 +851,18 @@ itimer_callout(void *arg)
 	} else {
 		getnanotime(&now);
 	}
+
 	backwards = (timespeccmp(&it->it_time.it_value, &now, >));
-	timespecadd(&it->it_time.it_value, &it->it_time.it_interval, &next);
+
+	/* Nonnegative interval guaranteed by itimerfix.  */
+	KASSERT(it->it_time.it_interval.tv_sec >= 0);
+	KASSERT(it->it_time.it_interval.tv_nsec >= 0);
+
 	/* Handle the easy case of non-overflown timers first. */
-	if (!backwards && timespeccmp(&next, &now, >)) {
+	if (!backwards &&
+	    timespecaddok(&it->it_time.it_value, &it->it_time.it_interval)) {
+		timespecadd(&it->it_time.it_value, &it->it_time.it_interval,
+		    &next);
 		it->it_time.it_value = next;
 	} else {
 		now_ns = timespec2ns(&now);