diff --git a/src/libiso8601/100_types.h b/src/libiso8601/100_types.h
index f0d38d0..253dfa3 100644
--- a/src/libiso8601/100_types.h
+++ b/src/libiso8601/100_types.h
@@ -103,6 +103,22 @@ struct iso8601_details {
 
 
 
+/*! \brief Short period elapsed time.
+
+This structure contains the details of an elapsed time period, split into seconds and nanoseconds.
+None of its components can ever be negative. This is only capable of representing short (compared to
+struct iso8601_date's range) periods of up to approximately 136 years.
+
+*/
+struct iso8601_elapsed {
+    /* number of whole days elapsed */
+    uint32_t day;
+    uint32_t sec;
+    uint32_t nsec;
+};
+
+
+
 /* 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/libiso8601/200_parser.c b/src/libiso8601/200_parser.c
index 82927d2..8a8aba9 100644
--- a/src/libiso8601/200_parser.c
+++ b/src/libiso8601/200_parser.c
@@ -37,6 +37,7 @@ int iso8601_parse(const char* str, struct iso8601_date* earliest, struct iso8601
     int num = 0, neg = 0, dig = 0, tz_neg = 0, nsec = -1, nsec_dig = -1, leap_sec_req = 0;
     int y = 0, m = -1, d = -1, w = -1, wd = -1, hour = -1, min = -1, sec = -1, tz_sec = 0;
     double frac;
+    struct iso8601_elapsed elapsed;
 
     if(earliest) memset(earliest, 0, sizeof(struct iso8601_date));
     if(latest) memset(latest, 0, sizeof(struct iso8601_date));
@@ -651,8 +652,16 @@ done:
     /* correct for timezone offset (note trickery to end up at 23:59:60 iff a leap second was 
      * requested), and ensure that any leap second requested was a valid leap second. */
     if(details) details->tz_sec = tz_sec;
-    if(tz_sec && earliest) iso8601_add_seconds(earliest, - leap_sec_req - tz_sec);
-    if(tz_sec && latest) iso8601_add_seconds(latest, - leap_sec_req - tz_sec);
+    if(tz_sec && earliest) {
+        elapsed.sec = leap_sec_req + tz_sec;
+        elapsed.nsec = 0;
+        iso8601_subtract_elapsed(earliest, &elapsed);
+    }
+    if(tz_sec && latest) {
+        elapsed.sec = leap_sec_req + tz_sec;
+        elapsed.nsec = 0;
+        iso8601_subtract_elapsed(latest, &elapsed);
+    }
     if(leap_sec_req) {
         ERROR_IF(earliest && earliest->sec != 86400);
         ERROR_IF(latest && latest->sec != 86400);
diff --git a/src/libiso8601/200_print.c b/src/libiso8601/200_print.c
index 0287820..69786a1 100644
--- a/src/libiso8601/200_print.c
+++ b/src/libiso8601/200_print.c
@@ -23,6 +23,7 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
     struct iso8601_date dttz;
     double frac;
     char* str_orig;
+    struct iso8601_elapsed elapsed;
 
     str_orig = str;
 
@@ -31,7 +32,11 @@ char* iso8601_print(char* str, int amt, const struct iso8601_date* date,
 
     // adjust output for timezone
     dttz = *date;
-    if(details->tz_sec) iso8601_add_seconds(&dttz, details->tz_sec);
+    if(details->tz_sec) {
+        elapsed.sec = details ? details->tz_sec : 0;
+        elapsed.nsec = 0;
+        iso8601_add_elapsed(&dttz, &elapsed);
+    }
 
     // determine whether or not to force extended output
     iso8601_to_cal(&y, &m, &d, &dttz);
diff --git a/src/libiso8601/400_calc.h b/src/libiso8601/400_calc.h
new file mode 100644
index 0000000..11efb70
--- /dev/null
+++ b/src/libiso8601/400_calc.h
@@ -0,0 +1,151 @@
+/* libiso8601/src/libiso8601/400_calc.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 manip Conversion and calculation routines.
+
+This set of functions is useful for converting dates and times into formats understood by humans,
+and vice versa.
+
+*/
+/*!@{*/
+
+
+
+/*! \brief Is year a leap year?
+
+\param year Year to examine.
+\retval 0 if not leap year.
+\retval non-0 if leap year.
+
+Returns non-0 if \a year is a leap year.
+
+*/
+int iso8601_isleap(int year);
+
+
+
+/*! \brief Convert date to calendar format.
+
+\param[out] year Calendar year.
+\param[out] month Calendar month (1 to 12).
+\param[out] day Calendar day (starting from 1).
+\param date Date to convert.
+
+*/
+void iso8601_to_cal(int* year, int* month, int* day, const struct iso8601_date* date);
+
+
+
+/*! \brief Convert date from calendar format.
+
+\param[out] date Output date.
+\param year Calendar year.
+\param month Calendar month (1 to 12).
+\param day Calenday day (starting from 1).
+\retval 0 on success.
+\retval -1 on error (and see \a errno).
+
+Converts the date specified in \a year, \a month and \a day into \a date. Does not touch the \a sec
+or \a nsec time members of \a date.
+
+*/
+int iso8601_from_cal(struct iso8601_date* date, int year, int month, int day);
+
+
+
+/*! \brief Convert date to ordinal format.
+
+\param[out] year Calendar year.
+\param[out] oday Ordinal day (from 1).
+\param date Date to convert.
+
+*/
+void iso8601_to_ord(int* year, int* oday, const struct iso8601_date* date);
+
+
+
+/*! \brief Convert date from ordinal format.
+
+\param[out] date Output date.
+\param year Calendar year.
+\param day Ordinal day (from 1).
+\retval 0 on success.
+\retval -1 on error (and see \a errno).
+
+Converts the date specified into \a year and \a oday into \a date. Does not touch the \a sec or
+\a nsec time members of \a date.
+
+*/
+int iso8601_from_ord(struct iso8601_date* date, int year, int oday);
+
+
+
+/*! \brief Convert date to week format.
+
+\param[out] year Calendar year.
+\param[out] week ISO week number (from 1).
+\param[out] wday ISO week day number (1 = Monday, 7 = Sunday).
+\param date Date to convert.
+
+*/
+void iso8601_to_week(int* year, int* week, int* wday, const struct iso8601_date* date);
+
+
+
+/*! \brief Convert date from week format.
+
+\param [out] date Output date.
+\param year Calendar year.
+\param week ISO week number (from 1).
+\param wday ISO week day number (1 = Monday, 7 = Sunday).
+\retval 0 on success.
+\retval -1 on error (and see \a errno).
+
+*/
+int iso8601_from_week(struct iso8601_date* date, int year, int week, int wday);
+
+
+
+/*! \brief Convert time to wall clock format.
+
+\param[out] hour Output hour.
+\param[out] min Output minute.
+\param[out] sec Output second.
+\param date Date/time to convert.
+
+Converts the time stored in \a date into wall clock format, storing the result in \a hour, \a min
+and \a sec.
+
+*/
+void iso8601_to_clocktime(int* hour, int* min, int* sec, const struct iso8601_date* date);
+
+
+
+/*! \brief Convert time from wall clock format.
+
+\param[out] Output time.
+\param hour Hour.
+\param min Minute.
+\param sec Second.
+\retval 0 on success.
+\retval -1 on error (and see \a errno).
+
+Converts the time as specified by \a hour, \a min and \a sec, storing the result in \a date. Does
+not touch the \a day (date) member of \a date.
+
+*/
+int iso8601_from_clocktime(struct iso8601_date* date, int hour, int min, int sec);
+
+
+
+/*!@}*/
+/* 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/libiso8601/400_functions.h b/src/libiso8601/400_functions.h
deleted file mode 100644
index 2996eaa..0000000
--- a/src/libiso8601/400_functions.h
+++ /dev/null
@@ -1,33 +0,0 @@
-/* libiso8601/src/libiso8601/functions.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.
-*/
-
-/* calc.c */
-int iso8601_isleap(int year);
-void iso8601_to_cal(int* year, int* month, int* day, const struct iso8601_date* date);
-int iso8601_from_cal(struct iso8601_date* date, int year, int month, int day);
-void iso8601_to_ord(int* year, int* oday, const struct iso8601_date* date);
-int iso8601_from_ord(struct iso8601_date* date, int year, int oday);
-void iso8601_to_week(int* year, int* week, int* wday, const struct iso8601_date* date);
-int iso8601_from_week(struct iso8601_date* date, int year, int week, int wday);
-void iso8601_to_clocktime(int* hour, int* min, int* sec, const struct iso8601_date* date);
-int iso8601_from_clocktime(struct iso8601_date* date, int hour, int min, int sec);
-
-/* manip.c */
-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);
-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
-*/
diff --git a/src/libiso8601/manip.c b/src/libiso8601/400_manip.c
similarity index 50%
rename from src/libiso8601/manip.c
rename to src/libiso8601/400_manip.c
index 34873ab..1ab5fd9 100644
--- a/src/libiso8601/manip.c
+++ b/src/libiso8601/400_manip.c
@@ -1,4 +1,4 @@
-/* libiso8601/src/libiso8601/manip.c
+/* libiso8601/src/libiso8601/400_manip.c
  *
  *  (c)2006, Laurence Withers, <l@lwithers.me.uk>.
  *  Released under the GNU GPLv2. See file COPYING or
@@ -38,51 +38,16 @@ 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_elapsed(struct iso8601_date* date, const struct iso8601_elapsed* per)
 {
-    int sday;
-    ldiv_t qr;
-
-    sday = date->day;
-    qr = ldiv(seconds + date->sec, 86400);
-    date->day += qr.quot;
-
-    date->sec = qr.rem;
-    if(date->sec < 0) {
-        --date->day;
-        date->sec += 86400;
-    }
-
-    /* leap second correction */
-    if(sday != date->day) {
-        date->sec += _leap_elapsed_day(sday, date->day);
-        if(date->sec < 0) {
-            --date->day;
-            date->sec += iso8601_seconds_leap(date);
-        } else if(date->sec >= iso8601_seconds_leap(date)) {
-            date->sec -= iso8601_seconds_leap(date);
-            ++date->day;
-        }
-    }
+    /* TODO */
 }
 
 
 
-void iso8601_add_nanoseconds(struct iso8601_date* date, long nsec)
+void iso8601_subtract_elapsed(struct iso8601_date* date, const struct iso8601_elapsed* per)
 {
-    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);
+    /* TODO */
 }
 
 
diff --git a/src/libiso8601/400_manip.h b/src/libiso8601/400_manip.h
new file mode 100644
index 0000000..0b75666
--- /dev/null
+++ b/src/libiso8601/400_manip.h
@@ -0,0 +1,80 @@
+/* libiso8601/src/libiso8601/400_manip.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 manip Manipulation routines.
+
+This set of functions is useful for converting dates and times into formats understood by humans,
+and vice versa.
+
+*/
+/*!@{*/
+
+
+
+/*! \brief Comparison (less than).
+
+\param d1 First date to compare.
+\param d2 Second date to compare.
+\retval non-0 if \a d1 &lt; \a d2
+\retval 0 otherwise
+
+*/
+int iso8601_lt(const struct iso8601_date* d1, const struct iso8601_date* d2);
+
+
+
+/*! \brief Comparison (less than or equal to).
+
+\param d1 First date to compare.
+\param d2 Second date to compare.
+\retval non-0 if \a d1 &lte; \a d2
+\retval 0 otherwise
+
+*/
+int iso8601_lte(const struct iso8601_date* d1, const struct iso8601_date* d2);
+
+
+
+/*! \brief Comparison (equality).
+
+\param d1 First date to compare.
+\param d2 Second date to compare.
+\retval non-0 if \a d1 == \a d2
+\retval 0 otherwise
+
+*/
+int iso8601_eq(const struct iso8601_date* d1, const struct iso8601_date* d2);
+
+
+
+/*! \brief Add a period to a date.
+
+\param[in,out] date Date to modify.
+\param per Period to advance date/time by.
+
+*/
+void iso8601_add_elapsed(struct iso8601_date* date, const struct iso8601_elapsed* per);
+
+
+
+/*! \brief Subtract a period from a date.
+
+\param[in,out] date Date to modify.
+\param per Period to regress date/time by.
+
+*/
+void iso8601_subtract_elapsed(struct iso8601_date* date, const struct iso8601_elapsed* per);
+
+
+
+/*!@}*/
+/* options for text editors
+kate: replace-trailing-space-save true; space-indent true; tab-width 4;
+vim: expandtab:ts=4:sw=4
+*/