diff --git a/sys/kern/subr_time.c b/sys/kern/subr_time.c
index 8a2815fcdb43..9e657be37caf 100644
--- a/sys/kern/subr_time.c
+++ b/sys/kern/subr_time.c
@@ -207,6 +207,7 @@ inittimeleft(struct timespec *ts, struct timespec *sleepts)
 	if (itimespecfix(ts)) {
 		return -1;
 	}
+	KASSERT(ts->tv_sec >= 0);
 	getnanouptime(sleepts);
 	return 0;
 }
@@ -214,15 +215,23 @@ inittimeleft(struct timespec *ts, struct timespec *sleepts)
 int
 gettimeleft(struct timespec *ts, struct timespec *sleepts)
 {
-	struct timespec sleptts;
+	struct timespec now, sleptts;
+
+	KASSERT(ts->tv_sec >= 0);
 
 	/*
 	 * Reduce ts by elapsed time based on monotonic time scale.
 	 */
-	getnanouptime(&sleptts);
-	timespecadd(ts, sleepts, ts);
+	getnanouptime(&now);
+	KASSERT(timespeccmp(sleepts, &now, <=));
+	timespecsub(&now, sleepts, &sleptts);
+	*sleepts = now;
+
+	if (timespeccmp(ts, &sleptts, <=)) { /* timed out */
+		timespecclear(ts);
+		return 0;
+	}
 	timespecsub(ts, &sleptts, ts);
-	*sleepts = sleptts;
 
 	return tstohz(ts);
 }