diff --git a/src/libiso8601/400_manip.c b/src/libiso8601/400_manip.c
index f66c131..f766b18 100644
--- a/src/libiso8601/400_manip.c
+++ b/src/libiso8601/400_manip.c
@@ -50,7 +50,7 @@ void iso8601_add_elapsed(struct iso8601_date* date, const struct iso8601_elapsed
     date->sec += qr.rem - leapcorrect;
 
     date->nsec += per->nsec;
-    if(date->nsec > BILLION) {
+    if(date->nsec >= BILLION) {
         ++date->sec;
         date->nsec -= BILLION;
     }