Fixes precise_diff on full month.

pendulum/helpers.py

@@ -190,11 +190,17 @@ def precise_diff(d1, d2):
                 d_diff += d1.day
             else:
                 d_diff += days_in_last_month
-
-            m_diff -= 1
-        else:
-            # We have a full month, remove days
+        elif d_diff == days_in_month - days_in_last_month:
+            # We have exactly a full month
+            # We remove the days difference
+            # and add one to the months difference
             d_diff = 0
+            m_diff += 1
+        else:
+            # We have a full month
+            d_diff += days_in_last_month
+
+        m_diff -= 1
 
     if m_diff < 0:
         m_diff += 12

tests/test_helpers.py

@@ -37,6 +37,15 @@ def test_precise_diff(self):
             hours=20, minutes=54, seconds=47, microseconds=282310
         )
 
+        dt1 = datetime(2017, 2, 17, 16, 5, 45, 123456)
+        dt2 = datetime(2018, 2, 17, 16, 5, 45, 123256)
+
+        diff = precise_diff(dt1, dt2)
+        self.assert_diff(
+            diff,
+            months=11, days=30, hours=23, minutes=59, seconds=59, microseconds=999800
+        )
+
     def assert_diff(self, diff,
                     years=0, months=0, days=0,
                     hours=0, minutes=0, seconds=0, microseconds=0):