Gracefully handle long time intervals

lib/iris/coords.py

@@ -575,7 +575,13 @@ def __str__(self):
             fmt = '{cls}({points}{bounds}' \
                   ', standard_name={self.standard_name!r}' \
                   ', calendar={self.units.calendar!r}{other_metadata})'
-            points = self._str_dates(self.points)
+            if self.units.is_long_time_interval():
+                # A time unit with a long time interval ("months" or "years")
+                # cannot be converted to a date using `num2date` so gracefully
+                # fall back to printing points as numbers, not datetimes.
+                points = self.points
+            else:
+                points = self._str_dates(self.points)
             bounds = ''
             if self.bounds is not None:
                 bounds = ', bounds=' + self._str_dates(self.bounds)

lib/iris/tests/unit/coords/test_Coord.py

@@ -368,5 +368,29 @@ def test_writable_points(self):
             coord2.bounds[:] = 0
 
 
+class Test___str__(tests.IrisTest):
+
+    def test_short_time_interval(self):
+        coord = DimCoord([5], standard_name='time',
+                         units='days since 1970-01-01')
+        expected = ("DimCoord([1970-01-06 00:00:00], standard_name='time', "
+                    "calendar='gregorian')")
+        result = coord.__str__()
+        self.assertEqual(expected, result)
+
+    def test_long_time_interval(self):
+        coord = DimCoord([5], standard_name='time',
+                         units='years since 1970-01-01')
+        expected = "DimCoord([5], standard_name='time', calendar='gregorian')"
+        result = coord.__str__()
+        self.assertEqual(expected, result)
+
+    def test_non_time_unit(self):
+        coord = DimCoord([1.])
+        expected = repr(coord)
+        result = coord.__str__()
+        self.assertEqual(expected, result)
+
+
 if __name__ == '__main__':
     tests.main()