diff --git a/src/libiso8601/200_parser.c b/src/libiso8601/200_parser.c
index 26b3caa..82927d2 100644
--- a/src/libiso8601/200_parser.c
+++ b/src/libiso8601/200_parser.c
@@ -669,21 +669,11 @@ done:
 
 int iso8601_invalid(const struct iso8601_date* date)
 {
-    int num_ly = 0, i = 0;
-
-    if(date->nsec < 0 || date->nsec >= BILLION) return 0;
-
-    switch(date->sec) {
-    case 0 ... 86399:
-        return 1;
-
-    case 86400: /* 23:59:60 */
-        num_ly = sizeof(leap_second_days) / sizeof(int);
-        for(i = 0; i < num_ly; ++i) if(leap_second_days[i] == date->day) return 1;
-        return 0;
-    }
-
-    return 0;
+    return date->nsec < 0
+        || date->nsec >= BILLION
+        || date->sec < 0
+        || date->sec >= iso8601_seconds_leap(date)
+    ;
 }
 
 
diff --git a/src/libiso8601/200_print.c b/src/libiso8601/200_print.c
index 9866518..0287820 100644
--- a/src/libiso8601/200_print.c
+++ b/src/libiso8601/200_print.c
@@ -22,6 +22,9 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
     int y, m, d, ret = 0, extended;
     struct iso8601_date dttz;
     double frac;
+    char* str_orig;
+
+    str_orig = str;
 
     // use default details if none provided
     if(!details) details = &_default_details;
@@ -38,12 +41,12 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
     case iso8601_prec_year:
         if(y < 0) snprintf(str, amt, "%05d", y);
         else snprintf(str, amt, "%04d", y);
-        return str;
+        return str_orig;
 
     case iso8601_prec_month:
         if(y < 0) snprintf(str, amt, "%05d-%02d", y, m);
         else snprintf(str, amt, extended ? "%04d-%02d" : "%04d%02d", y, m);
-        return str;
+        return str_orig;
 
     case iso8601_prec_day:
         if(y < 0) ret = snprintf(str, amt, "%05d-%02d-%02d", y, m, d);
@@ -61,7 +64,7 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
         extended = y < 0 || y > 9999 || details->extended; // ISO year is different
         if(y < 0) snprintf(str, amt, "%05d-W%02d", y, m);
         else snprintf(str, amt, extended ? "%04d-W%02d" : "%04dW%02d", y, m);
-        return str;
+        return str_orig;
 
     case iso8601_prec_wday:
         iso8601_to_week(&y, &m, &d, &dttz);
@@ -71,7 +74,7 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
         break;
     }
 
-    if(ret < 1 || ret >= amt) return str;
+    if(ret < 1 || ret >= amt) return str_orig;
     str += ret;
     amt -= ret;
 
@@ -92,7 +95,7 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
 
     switch(details->time_prec) {
     case iso8601_prec_none:
-        return str;
+        return str_orig;
 
     case iso8601_prec_hour:
         ret = snprintf(str, amt, "T%02d", y);
@@ -122,14 +125,14 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
         break;
     }
 
-    if(ret < 1 || ret >= amt) return str;
+    if(ret < 1 || ret >= amt) return str_orig;
     str += ret;
     amt -= ret;
 
     if(details->tz_sec) {
         if(!--amt) {
             *str = 0;
-            return str;
+            return str_orig;
         }
         if(details->tz_sec < 0) {
             *str++ = '-';
@@ -152,6 +155,8 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
         *str++ = 'Z';
         if(amt > 1) *str = 0;
     }
+    
+    return str_orig;
 }
 
 
diff --git a/src/libiso8601/400_leap.h b/src/libiso8601/400_leap.h
new file mode 100644
index 0000000..4d64522
--- /dev/null
+++ b/src/libiso8601/400_leap.h
@@ -0,0 +1,57 @@
+/* libiso8601/src/libiso8601/400_leap.h
+ *
+ *  (c)2006, Laurence Withers, <l@lwithers.me.uk>.
+ *  Released under the GNU GPLv2. See file COPYING or
+ *  http://www.gnu.org/copyleft/gpl.html for details.
+*/
+
+
+
+/*! \defgroup leap Leap second support.
+
+A set of functions for explicitly dealing with leap seconds.
+
+*/
+/*!@{*/
+
+
+
+/*! \brief Return number of seconds in day, taking leap seconds into account.
+
+\param date The date to return the number of seconds for.
+\retval 86399 day with a negative leap second.
+\retval 86400 day with no leap second.
+\retval 86401 day with one leap second.
+
+Returns the duration of a day \a date, in seconds. This function takes leap seconds into account and
+may be used to determine if a day contains a leap second or not.
+
+\note There have not yet been any days requiring a negative leap second, so at present 86399 will
+    never be returned.
+
+*/
+int iso8601_seconds_leap(const struct iso8601_date* date);
+
+
+
+/*! \brief Return number of leap seconds elapsed between two days.
+
+\param start The start date.
+\param end The end date.
+\returns Number of leap seconds elapsed.
+
+Computes the number of leap seconds that have elapsed between two days. Note that this is the sum of
+such leap seconds, so it will be 0 if (for example) there is one positive leap second and one 
+negative leap second. The ordering of the dates is important; if \a start is after \a end, then the
+value returned will be negative (for positive leap seconds).
+
+*/
+int iso8601_leap_elapsed(const struct iso8601_date* start, const struct iso8601_date* end);
+
+
+
+/*!@}*/
+/* options for text editors
+kate: replace-trailing-space-save true; space-indent true; tab-width 4;
+vim: expandtab:ts=4:sw=4
+*/