diff --git a/src/libiso8601/400_manip.c b/src/libiso8601/400_manip.c
index d9489d1..92185ea 100644
--- a/src/libiso8601/400_manip.c
+++ b/src/libiso8601/400_manip.c
@@ -48,16 +48,22 @@ void iso8601_add_elapsed(struct iso8601_date* date, const struct iso8601_elapsed
     leapcorrect = _leap_elapsed_day(date->day, date->day + qr.quot);
     date->day += qr.quot;
     date->sec += qr.rem - leapcorrect;
-    if(date->sec < 0) {
-        --date->day;
-        date->sec += iso8601_seconds_leap(date);
-    }
 
     date->nsec += per->nsec;
     if(date->nsec > BILLION) {
         ++date->sec;
         date->nsec -= BILLION;
     }
+
+    if(date->sec < 0) {
+        --date->day;
+        date->sec += iso8601_seconds_leap(date);
+    }
+
+    if(date->sec >= iso8601_seconds_leap(date)) {
+        date->sec -= iso8601_seconds_leap(date);
+        ++date->day;
+    }
 }
 
 
@@ -73,6 +79,7 @@ void iso8601_subtract_elapsed(struct iso8601_date* date, const struct iso8601_el
     date->day -= qr.quot;
     date->sec -= qr.rem - leapcorrect;
     date->nsec -= per->nsec;
+
     if(date->nsec < 0) {
         --date->sec;
         date->nsec += BILLION;
@@ -82,6 +89,7 @@ void iso8601_subtract_elapsed(struct iso8601_date* date, const struct iso8601_el
         --date->day;
         date->sec += iso8601_seconds_leap(date);
     }
+
     if(date->sec >= iso8601_seconds_leap(date)) {
         date->sec -= iso8601_seconds_leap(date);
         ++date->day;