Fixes wrong overlapping check in _can_fast_union.

To use the fast_union method it is not enough to find an overlap, the
indices also have to be aligned.

doc/source/release.rst

@@ -462,6 +462,7 @@ Bug Fixes
   - Fixed ``copy()`` to shallow copy axes/indices as well and thereby keep
     separate metadata. (:issue:`4202`, :issue:`4830`)
   - Fixed skiprows option in Python parser for read_csv (:issue:`4382`)
+  - Fixed wrong check for overlapping in ``DatetimeIndex.union`` (:issue:`4564`)
 
 pandas 0.12.0
 -------------

pandas/tseries/index.py

@@ -986,11 +986,11 @@ def _can_fast_union(self, other):
         else:
             left, right = other, self
 
-        left_end = left[-1]
         right_start = right[0]
+        left_end = left[-1]
 
         # Only need to "adjoin", not overlap
-        return (left_end + offset) >= right_start
+        return (right_start == left_end + offset) or right_start in left
 
     def _fast_union(self, other):
         if len(other) == 0:

pandas/tseries/tests/test_timeseries.py

@@ -1859,6 +1859,15 @@ def test_union_bug_1745(self):
         exp = DatetimeIndex(sorted(set(list(left)) | set(list(right))))
         self.assert_(result.equals(exp))
 
+    def test_union_bug_4564(self):
+        from pandas import DateOffset
+        left = date_range("2013-01-01", "2013-02-01")
+        right = left + DateOffset(minutes=15)
+
+        result = left.union(right)
+        exp = DatetimeIndex(sorted(set(list(left)) | set(list(right))))
+        self.assert_(result.equals(exp))
+
     def test_intersection_bug_1708(self):
         from pandas import DateOffset
         index_1 = date_range('1/1/2012', periods=4, freq='12H')