diff --git a/src/libiso8601/TopSource.c b/src/libiso8601/TopSource.c
index 2045dbf..102cc8d 100644
--- a/src/libiso8601/TopSource.c
+++ b/src/libiso8601/TopSource.c
@@ -7,13 +7,13 @@
 
 #include "iso8601.h"
 
-// Below are all the includes used throughout the library.
+/* Below are all the includes used throughout the library. */
 #include <errno.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-
-#include <stdio.h>
+#define BILLION (1000000000L)
 
 /* options for text editors
 kate: replace-trailing-space-save true; space-indent true; tab-width 4;
diff --git a/src/libiso8601/functions.h b/src/libiso8601/functions.h
index de00936..7238fd1 100644
--- a/src/libiso8601/functions.h
+++ b/src/libiso8601/functions.h
@@ -71,6 +71,7 @@ int iso8601_lt(const struct iso8601_date* d1, const struct iso8601_date* d2);
 int iso8601_lte(const struct iso8601_date* d1, const struct iso8601_date* d2);
 int iso8601_eq(const struct iso8601_date* d1, const struct iso8601_date* d2);
 void iso8601_add_seconds(struct iso8601_date* date, long seconds);
+void iso8601_add_nanoseconds(struct iso8601_date* date, long nano);
 
 /* leap.c */
 int iso8601_seconds_leap(const struct iso8601_date* date);
diff --git a/src/libiso8601/leap.c b/src/libiso8601/leap.c
index 4ea0bcf..4893473 100644
--- a/src/libiso8601/leap.c
+++ b/src/libiso8601/leap.c
@@ -50,7 +50,7 @@ int iso8601_valid_leap(const struct iso8601_date* date)
 {
     int num_ly = 0, i = 0;
 
-    if(date->nsec < 0 || date->nsec >= 1000000000) return 0;
+    if(date->nsec < 0 || date->nsec >= BILLION) return 0;
 
     switch(date->sec) {
     case 0 ... 86399:
diff --git a/src/libiso8601/manip.c b/src/libiso8601/manip.c
index e611a1d..34873ab 100644
--- a/src/libiso8601/manip.c
+++ b/src/libiso8601/manip.c
@@ -68,6 +68,25 @@ void iso8601_add_seconds(struct iso8601_date* date, long seconds)
 
 
 
+void iso8601_add_nanoseconds(struct iso8601_date* date, long nsec)
+{
+    ldiv_t qr;
+
+    qr = ldiv(nsec, BILLION);
+    date->nsec += qr.rem;
+    if(date->nsec > BILLION) {
+        date->nsec -= BILLION;
+        ++qr.quot;
+    } else if(date->nsec < 0) {
+        date->nsec += BILLION;
+        --qr.quot;
+    }
+
+    iso8601_add_seconds(date, qr.quot);
+}
+
+
+
 /* options for text editors
 kate: replace-trailing-space-save true; space-indent true; tab-width 4;
 vim: expandtab:ts=4:sw=4
diff --git a/src/libiso8601/parser.c b/src/libiso8601/parser.c
index 40dc41f..5a9b2ef 100644
--- a/src/libiso8601/parser.c
+++ b/src/libiso8601/parser.c
@@ -46,7 +46,7 @@ int iso8601_parse(const char* str, struct iso8601_date* earliest, struct iso8601
     ++dig; \
     num *= 10; \
     num += ch - '0'; \
-    if(num > 1000000000) { \
+    if(num > BILLION) { \
         return -1; \
     } \
 }while(0)