Truncated timepoint addition returns next matching point instead of the original point if the two already match

Author: MetRonnie

metomi/isodatetime/data.py

@@ -1572,7 +1572,7 @@ def find_next_month_and_day(
     def find_next_month_and_day(
         self, month: Optional[int], day: Optional[int]
     ) -> 'TimePoint':
-        """Return the next TimePoint after this one (inclusive) that has the
+        """Return the next TimePoint after this one that has the
         same month and/or day as specified.
 
         Args:
@@ -1594,18 +1594,25 @@ def _next_month_and_day(
         for i, year in enumerate(years_to_check):
             self._year = year
             if month:
-                if month >= self._month_of_year and (
-                    day is None or
-                    self._day_of_month <= day <= get_days_in_month(month, year)
-                ):
-                    self._month_of_year = month
-                    self._day_of_month = day or 1
-                    return
+                if day is None:
+                    if month > self._month_of_year:
+                        self._month_of_year = month
+                        self._day_of_month = 1
+                        return
+                else:
+                    if month >= self._month_of_year and (
+                        self._day_of_month < day <= get_days_in_month(
+                            month, year
+                        )
+                    ):
+                        self._month_of_year = month
+                        self._day_of_month = day
+                        return
             else:
                 for month_ in range(
                     self._month_of_year, CALENDAR.MONTHS_IN_YEAR + 1
                 ):
-                    if self._day_of_month <= day <= get_days_in_month(
+                    if self._day_of_month < day <= get_days_in_month(
                         month_, year
                     ):
                         self._month_of_year = month_

metomi/isodatetime/tests/test_01.py

@@ -720,13 +720,18 @@ def tp_add_param(timepoint, other, expected):
         tp_add_param(
             data.TimePoint(year=2000, day_of_month=15),
             data.TimePoint(month_of_year=1, day_of_month=15, truncated=True),
-            data.TimePoint(year=2000, day_of_month=15),
+            data.TimePoint(year=2001, day_of_month=15),
         ),
         tp_add_param(
             data.TimePoint(year=2000, day_of_month=15),
             data.TimePoint(month_of_year=1, day_of_month=14, truncated=True),
             data.TimePoint(year=2001, day_of_month=14),
         ),
+        tp_add_param(
+            data.TimePoint(year=2000, day_of_month=31),
+            data.TimePoint(day_of_month=31, truncated=True),
+            data.TimePoint(year=2000, month_of_year=3, day_of_month=31),
+        ),
         tp_add_param(
             data.TimePoint(year=2000, day_of_month=15),
             data.TimePoint(day_of_month=14, truncated=True),
@@ -769,6 +774,11 @@ def test_timepoint_add(
             '-11-02',
             data.TimePoint(year=2011, month_of_year=2, day_of_month=1)
         ),
+        tp_add_param(
+            '2014-04-10T00Z',
+            '-14-04',
+            data.TimePoint(year=2114, month_of_year=4, day_of_month=1)
+        ),
     ]
 )
 def test_timepoint_add__extra(