diff --git a/src/libiso8601/100_leap.c b/src/libiso8601/100_leap.c
index cf2a900..851d274 100644
--- a/src/libiso8601/100_leap.c
+++ b/src/libiso8601/100_leap.c
@@ -57,7 +57,7 @@ static int _leap_elapsed_day(int sday, int eday)
         if(eday <= leap_second_days[epos]) break;
     }
 
-    return spos - epos;
+    return epos - spos;
 }
 
 
diff --git a/src/libiso8601/400_manip.c b/src/libiso8601/400_manip.c
index 1ab5fd9..d9489d1 100644
--- a/src/libiso8601/400_manip.c
+++ b/src/libiso8601/400_manip.c
@@ -40,14 +40,52 @@ int iso8601_eq(const struct iso8601_date* d1, const struct iso8601_date* d2)
 
 void iso8601_add_elapsed(struct iso8601_date* date, const struct iso8601_elapsed* per)
 {
-    /* TODO */
+    div_t qr;
+    int leapcorrect;
+
+    /* work out number to advance `day' by */
+    qr = div(per->sec, 86400);
+    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;
+    }
 }
 
 
 
 void iso8601_subtract_elapsed(struct iso8601_date* date, const struct iso8601_elapsed* per)
 {
-    /* TODO */
+    div_t qr;
+    int leapcorrect;
+
+    /* work out number to advance `day' by */
+    qr = div(per->sec, 86400);
+    leapcorrect = _leap_elapsed_day(date->day - qr.quot, date->day);
+    date->day -= qr.quot;
+    date->sec -= qr.rem - leapcorrect;
+    date->nsec -= per->nsec;
+    if(date->nsec < 0) {
+        --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;
+    }
 }
 
 
diff --git a/src/tests/leap.c b/src/tests/leap.c
index 0f36def..11cf7a9 100644
--- a/src/tests/leap.c
+++ b/src/tests/leap.c
@@ -30,6 +30,7 @@ int leap_diff(const char* s1, const char* s2, int* diff)
 int regression1(void)
 {
     struct iso8601_date d;
+    struct iso8601_elapsed elapsed;
     char s1[30], s2[30], s3[30];
     int ret = 0, oday;
 
@@ -39,11 +40,13 @@ int regression1(void)
     d.nsec = 0;
     iso8601_print(s1, sizeof(s1), &d, 0);
 
-    iso8601_add_seconds(&d, 86400);
+    elapsed.sec = 86400;
+    elapsed.nsec = 0;
+    iso8601_add_elapsed(&d, &elapsed);
     iso8601_print(s2, sizeof(s2), &d, 0);
     ret |= (d.day != oday) | (d.sec != 86400);
 
-    iso8601_add_seconds(&d, 86400);
+    iso8601_add_elapsed(&d, &elapsed);
     iso8601_print(s3, sizeof(s3), &d, 0);
     ret |= (d.day != (oday + 1)) | (d.sec != 86399);
 
@@ -58,6 +61,7 @@ int regression2(void)
 {
     struct iso8601_date d;
     char s1[30], s2[30], s3[30];
+    struct iso8601_elapsed elapsed;
     int ret = 0, oday;
 
     iso8601_from_cal(&d, 2005, 12, 31);
@@ -66,11 +70,13 @@ int regression2(void)
     d.nsec = 0;
     iso8601_print(s1, sizeof(s1), &d, 0);
 
-    iso8601_add_seconds(&d, 86400);
+    elapsed.sec = 86400;
+    elapsed.nsec = 0;
+    iso8601_add_elapsed(&d, &elapsed);
     iso8601_print(s2, sizeof(s2), &d, 0);
     ret |= (d.day != (oday + 1)) | (d.sec != 86398);
 
-    iso8601_add_seconds(&d, 86400);
+    iso8601_add_elapsed(&d, &elapsed);
     iso8601_print(s3, sizeof(s3), &d, 0);
     ret |= (d.day != (oday + 2)) | (d.sec != 86398);
 
@@ -85,6 +91,7 @@ int regression3(void)
 {
     struct iso8601_date d;
     char s1[30], s2[30], s3[30];
+    struct iso8601_elapsed elapsed;
     int ret = 0, oday;
 
     iso8601_from_cal(&d, 2005, 12, 31);
@@ -93,11 +100,13 @@ int regression3(void)
     d.nsec = 0;
     iso8601_print(s1, sizeof(s1), &d, 0);
 
-    iso8601_add_seconds(&d, 86400);
+    elapsed.sec = 86400;
+    elapsed.nsec = 0;
+    iso8601_add_elapsed(&d, &elapsed);
     iso8601_print(s2, sizeof(s2), &d, 0);
     ret |= (d.day != (oday + 1)) | (d.sec != 86399);
 
-    iso8601_add_seconds(&d, 86400);
+    iso8601_add_elapsed(&d, &elapsed);
     iso8601_print(s3, sizeof(s3), &d, 0);
     ret |= (d.day != (oday + 2)) | (d.sec != 86399);
 
@@ -112,6 +121,7 @@ int regression4(void)
 {
     struct iso8601_date d;
     char s1[30], s2[30], s3[30];
+    struct iso8601_elapsed elapsed;
     int ret = 0, oday;
 
     iso8601_from_cal(&d, 2006, 1, 1);
@@ -120,11 +130,13 @@ int regression4(void)
     d.nsec = 0;
     iso8601_print(s1, sizeof(s1), &d, 0);
 
-    iso8601_add_seconds(&d, -86400);
+    elapsed.sec = 86400;
+    elapsed.nsec = 0;
+    iso8601_subtract_elapsed(&d, &elapsed);
     iso8601_print(s2, sizeof(s2), &d, 0);
     ret |= (d.day != (oday - 1)) | (d.sec != 86400);
 
-    iso8601_add_seconds(&d, -86400);
+    iso8601_subtract_elapsed(&d, &elapsed);
     iso8601_print(s3, sizeof(s3), &d, 0);
     ret |= (d.day != (oday - 1)) | (d.sec != 0);
 
@@ -139,6 +151,7 @@ int regression5(void)
 {
     struct iso8601_date d;
     char s1[30], s2[30], s3[30];
+    struct iso8601_elapsed elapsed;
     int ret = 0, oday;
 
     iso8601_from_cal(&d, 2006, 1, 2);
@@ -147,11 +160,13 @@ int regression5(void)
     d.nsec = 0;
     iso8601_print(s1, sizeof(s1), &d, 0);
 
-    iso8601_add_seconds(&d, -86400);
+    elapsed.sec = 86400;
+    elapsed.nsec = 0;
+    iso8601_subtract_elapsed(&d, &elapsed);
     iso8601_print(s2, sizeof(s2), &d, 0);
     ret |= (d.day != (oday - 1)) | (d.sec != 86398);
 
-    iso8601_add_seconds(&d, -86400);
+    iso8601_subtract_elapsed(&d, &elapsed);
     iso8601_print(s3, sizeof(s3), &d, 0);
     ret |= (d.day != (oday - 2)) | (d.sec != 86399);
 
@@ -166,6 +181,7 @@ int regression6(void)
 {
     struct iso8601_date d;
     char s1[30], s2[30], s3[30];
+    struct iso8601_elapsed elapsed;
     int ret = 0, oday;
 
     iso8601_from_cal(&d, 2006, 1, 2);
@@ -174,11 +190,13 @@ int regression6(void)
     d.nsec = 0;
     iso8601_print(s1, sizeof(s1), &d, 0);
 
-    iso8601_add_seconds(&d, -86400);
+    elapsed.sec = 86400;
+    elapsed.nsec = 0;
+    iso8601_subtract_elapsed(&d, &elapsed);
     iso8601_print(s2, sizeof(s2), &d, 0);
     ret |= (d.day != (oday - 1)) | (d.sec != 86399);
 
-    iso8601_add_seconds(&d, -86400);
+    iso8601_subtract_elapsed(&d, &elapsed);
     iso8601_print(s3, sizeof(s3), &d, 0);
     ret |= (d.day != (oday - 2)) | (d.sec != 86400);