From be2b6dd7ff099b8ea219518cb7c72c272864c1ee Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Thu, 30 Aug 2018 23:43:59 -0700
Subject: [PATCH 01/11] ENH: Add is_dst method to DatetimeIndex

---
 doc/source/api.rst                            |  2 ++
 doc/source/whatsnew/v0.24.0.txt               |  1 +
 pandas/core/indexes/datetimelike.py           |  2 +-
 pandas/core/indexes/datetimes.py              | 34 ++++++++++++++++++-
 .../tests/indexes/datetimes/test_timezones.py | 11 ++++++
 pandas/tests/series/test_datetime_values.py   |  3 +-
 6 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/doc/source/api.rst b/doc/source/api.rst
index 77d37ec2a7b2e..1b299d03f13f6 100644
--- a/doc/source/api.rst
+++ b/doc/source/api.rst
@@ -582,6 +582,7 @@ These can be accessed like ``Series.dt.<property>``.
    Series.dt.to_pydatetime
    Series.dt.tz_localize
    Series.dt.tz_convert
+   Series.dt.is_dst
    Series.dt.normalize
    Series.dt.strftime
    Series.dt.round
@@ -1778,6 +1779,7 @@ Time-specific operations
    DatetimeIndex.snap
    DatetimeIndex.tz_convert
    DatetimeIndex.tz_localize
+   DatetimeIndex.is_dst
    DatetimeIndex.round
    DatetimeIndex.floor
    DatetimeIndex.ceil
diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index be16806cb4d1e..3de38b2b3dcf9 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -184,6 +184,7 @@ Other Enhancements
 - :class:`DatetimeIndex` gained :attr:`DatetimeIndex.timetz` attribute. Returns local time with timezone information. (:issue:`21358`)
 - :class:`Resampler` now is iterable like :class:`GroupBy` (:issue:`15314`).
 - :ref:`Series.resample` and :ref:`DataFrame.resample` have gained the :meth:`Resampler.quantile` (:issue:`15023`).
+- :class:`DatetimeIndex` has gained a `is_dst` method.
 
 .. _whatsnew_0240.api_breaking:
 
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 3f8c07fe7cd21..c6ef015e1f54a 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -284,7 +284,7 @@ def _ensure_localized(self, result):
         if getattr(self, 'tz', None) is not None:
             if not isinstance(result, ABCIndexClass):
                 result = self._simple_new(result)
-            result = result.tz_localize(self.tz)
+            result = result.tz_localize(self.tz, ambiguous=self.is_dst())
         return result
 
     def _box_values_as_index(self):
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 019aad4941d26..2358b1f2fb33d 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -266,7 +266,7 @@ def _add_comparison_methods(cls):
     _datetimelike_methods = ['to_period', 'tz_localize',
                              'tz_convert',
                              'normalize', 'strftime', 'round', 'floor',
-                             'ceil', 'month_name', 'day_name']
+                             'ceil', 'month_name', 'day_name', 'is_dst']
 
     _is_numeric_dtype = False
     _infer_as_myclass = True
@@ -443,6 +443,38 @@ def tz(self, value):
         raise AttributeError("Cannot directly set timezone. Use tz_localize() "
                              "or tz_convert() as appropriate")
 
+    def is_dst(self):
+        """
+        Returns an Index of booleans indicating if each corresponding timestamp
+        is in daylight savings time.
+
+        If the DatetimeIndex does not have a timezone, returns an Index
+        who's values are all False.
+
+        Returns
+        -------
+        Index
+            True if the timestamp is in daylight savings time else False
+
+        Example
+        -------
+        >>> dti = pd.date_range('2018-11-04', periods=4, freq='H',
+                                tz='US/Pacific')
+
+        >>> dti
+        DatetimeIndex(['2018-11-04 00:00:00-07:00',
+                       '2018-11-04 01:00:00-07:00',
+                       '2018-11-04 01:00:00-08:00',
+                       '2018-11-04 02:00:00-08:00'],
+                      dtype='datetime64[ns, US/Pacific]', freq='H')
+
+        >>> dti.is_dst()
+        Index([True, True, False, False], dtype='object')
+        """
+        if self.tz is None:
+            return Index([False] * len(self))
+        return Index([bool(ts.dst()) for ts in self])
+
     @property
     def size(self):
         # TODO: Remove this when we have a DatetimeTZArray
diff --git a/pandas/tests/indexes/datetimes/test_timezones.py b/pandas/tests/indexes/datetimes/test_timezones.py
index 95531b2d7a7ae..55eb0468d7401 100644
--- a/pandas/tests/indexes/datetimes/test_timezones.py
+++ b/pandas/tests/indexes/datetimes/test_timezones.py
@@ -1012,6 +1012,17 @@ def test_iteration_preserves_nanoseconds(self, tz):
         for i, ts in enumerate(index):
             assert ts == index[i]
 
+    def test_is_dst(self):
+        dti = pd.date_range('2018-11-04', periods=4, freq='H', tz='US/Pacific')
+        result = dti.is_dst()
+        expected = Index([True, True, False, False])
+        tm.assert_index_equal(result, expected)
+
+        dti_naive = dti.tz_localize(None)
+        result = dti_naive.is_dst()
+        expected = Index([False] * 4)
+        tm.assert_index_equal(result, expected)
+
 
 class TestDateRange(object):
     """Tests for date_range with timezones"""
diff --git a/pandas/tests/series/test_datetime_values.py b/pandas/tests/series/test_datetime_values.py
index 5b45c6003a005..3b3fe2983d61a 100644
--- a/pandas/tests/series/test_datetime_values.py
+++ b/pandas/tests/series/test_datetime_values.py
@@ -37,7 +37,8 @@ def test_dt_namespace_accessor(self):
         ok_for_dt = DatetimeIndex._datetimelike_ops
         ok_for_dt_methods = ['to_period', 'to_pydatetime', 'tz_localize',
                              'tz_convert', 'normalize', 'strftime', 'round',
-                             'floor', 'ceil', 'day_name', 'month_name']
+                             'floor', 'ceil', 'day_name', 'month_name',
+                             'is_dst']
         ok_for_td = TimedeltaIndex._datetimelike_ops
         ok_for_td_methods = ['components', 'to_pytimedelta', 'total_seconds',
                              'round', 'floor', 'ceil']

From 18f8611c34dcdc73138eb43fcbeb6b613f0fc58b Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Fri, 31 Aug 2018 00:20:55 -0700
Subject: [PATCH 02/11] Address merge issue

---
 pandas/_libs/tslibs/nattype.pyx             |  1 -
 pandas/tests/reshape/merge/test_merge.py    | 24 +++++++
 pandas/tests/series/test_datetime_values.py | 75 +++++++++++----------
 3 files changed, 63 insertions(+), 37 deletions(-)

diff --git a/pandas/_libs/tslibs/nattype.pyx b/pandas/_libs/tslibs/nattype.pyx
index 08d9128ff660c..c0263b747b081 100644
--- a/pandas/_libs/tslibs/nattype.pyx
+++ b/pandas/_libs/tslibs/nattype.pyx
@@ -353,7 +353,6 @@ class NaTType(_NaT):
     strptime = _make_error_func('strptime', datetime)
     strftime = _make_error_func('strftime', datetime)
     isocalendar = _make_error_func('isocalendar', datetime)
-    dst = _make_error_func('dst', datetime)
     ctime = _make_error_func('ctime', datetime)
     time = _make_error_func('time', datetime)
     toordinal = _make_error_func('toordinal', datetime)
diff --git a/pandas/tests/reshape/merge/test_merge.py b/pandas/tests/reshape/merge/test_merge.py
index 42df4511578f1..2ec5c2b78a04f 100644
--- a/pandas/tests/reshape/merge/test_merge.py
+++ b/pandas/tests/reshape/merge/test_merge.py
@@ -601,6 +601,30 @@ def test_merge_on_datetime64tz(self):
         assert result['value_x'].dtype == 'datetime64[ns, US/Eastern]'
         assert result['value_y'].dtype == 'datetime64[ns, US/Eastern]'
 
+    def test_merge_datetime64tz_with_dst_transition(self):
+        # GH 18885
+        df1 = pd.DataFrame(pd.date_range(
+            '2017-10-29 01:00', periods=4, freq='H', tz='Europe/Madrid'),
+            columns=['date'])
+        df1['value'] = 1
+        df2 = pd.DataFrame([
+            pd.to_datetime('2017-10-29 03:00:00'),
+            pd.to_datetime('2017-10-29 04:00:00'),
+            pd.to_datetime('2017-10-29 05:00:00')
+        ],
+            columns=['date'])
+        df2['date'] = df2['date'].dt.tz_localize('UTC').dt.tz_convert(
+            'Europe/Madrid')
+        df2['value'] = 2
+        result = pd.merge(df1, df2, how='outer', on='date')
+        expected = pd.DataFrame({
+            'date': pd.date_range(
+                '2017-10-29 01:00', periods=7, freq='H', tz='Europe/Madrid'),
+            'value_x': [1] * 4 + [np.nan] * 3,
+            'value_y': [np.nan] * 4 + [2] * 3
+        })
+        assert_frame_equal(result, expected)
+
     def test_merge_non_unique_period_index(self):
         # GH #16871
         index = pd.period_range('2016-01-01', periods=16, freq='M')
diff --git a/pandas/tests/series/test_datetime_values.py b/pandas/tests/series/test_datetime_values.py
index 3b3fe2983d61a..9921d880ff78a 100644
--- a/pandas/tests/series/test_datetime_values.py
+++ b/pandas/tests/series/test_datetime_values.py
@@ -96,42 +96,6 @@ def compare(s, name):
             expected = Series(exp_values, index=s.index, name='xxx')
             tm.assert_series_equal(result, expected)
 
-        # round
-        s = Series(pd.to_datetime(['2012-01-01 13:00:00',
-                                   '2012-01-01 12:01:00',
-                                   '2012-01-01 08:00:00']), name='xxx')
-        result = s.dt.round('D')
-        expected = Series(pd.to_datetime(['2012-01-02', '2012-01-02',
-                                          '2012-01-01']), name='xxx')
-        tm.assert_series_equal(result, expected)
-
-        # round with tz
-        result = (s.dt.tz_localize('UTC')
-                   .dt.tz_convert('US/Eastern')
-                   .dt.round('D'))
-        exp_values = pd.to_datetime(['2012-01-01', '2012-01-01',
-                                     '2012-01-01']).tz_localize('US/Eastern')
-        expected = Series(exp_values, name='xxx')
-        tm.assert_series_equal(result, expected)
-
-        # floor
-        s = Series(pd.to_datetime(['2012-01-01 13:00:00',
-                                   '2012-01-01 12:01:00',
-                                   '2012-01-01 08:00:00']), name='xxx')
-        result = s.dt.floor('D')
-        expected = Series(pd.to_datetime(['2012-01-01', '2012-01-01',
-                                          '2012-01-01']), name='xxx')
-        tm.assert_series_equal(result, expected)
-
-        # ceil
-        s = Series(pd.to_datetime(['2012-01-01 13:00:00',
-                                   '2012-01-01 12:01:00',
-                                   '2012-01-01 08:00:00']), name='xxx')
-        result = s.dt.ceil('D')
-        expected = Series(pd.to_datetime(['2012-01-02', '2012-01-02',
-                                          '2012-01-02']), name='xxx')
-        tm.assert_series_equal(result, expected)
-
         # datetimeindex with tz
         s = Series(date_range('20130101', periods=5, tz='US/Eastern'),
                    name='xxx')
@@ -262,6 +226,45 @@ def get_dir(s):
             with pytest.raises(com.SettingWithCopyError):
                 s.dt.hour[0] = 5
 
+    @pytest.mark.parametrize('method, dates', [
+        ['round', ['2012-01-02', '2012-01-02', '2012-01-01']],
+        ['floor', ['2012-01-01', '2012-01-01', '2012-01-01']],
+        ['ceil', ['2012-01-02', '2012-01-02', '2012-01-02']]
+    ])
+    def test_dt_round(self, method, dates):
+        # round
+        s = Series(pd.to_datetime(['2012-01-01 13:00:00',
+                                   '2012-01-01 12:01:00',
+                                   '2012-01-01 08:00:00']), name='xxx')
+        result = getattr(s.dt, method)('D')
+        expected = Series(pd.to_datetime(dates), name='xxx')
+        tm.assert_series_equal(result, expected)
+
+    def test_dt_round_tz(self):
+        s = Series(pd.to_datetime(['2012-01-01 13:00:00',
+                                   '2012-01-01 12:01:00',
+                                   '2012-01-01 08:00:00']), name='xxx')
+
+        result = (s.dt.tz_localize('UTC')
+                   .dt.tz_convert('US/Eastern')
+                   .dt.round('D'))
+        exp_values = pd.to_datetime(['2012-01-01', '2012-01-01',
+                                     '2012-01-01']).tz_localize('US/Eastern')
+        expected = Series(exp_values, name='xxx')
+        tm.assert_series_equal(result, expected)
+
+        # GH 18946 round near DST
+        df1 = pd.DataFrame([
+            pd.to_datetime('2017-10-29 02:00:00+02:00', utc=True),
+            pd.to_datetime('2017-10-29 02:00:00+01:00', utc=True),
+            pd.to_datetime('2017-10-29 03:00:00+01:00', utc=True)
+        ],
+            columns=['date'])
+        df1['date'] = df1['date'].dt.tz_convert('Europe/Madrid')
+        result = df1.date.dt.floor('H')
+        expected = df1['date']
+        tm.assert_series_equal(result, expected)
+
     def test_dt_namespace_accessor_categorical(self):
         # GH 19468
         dti = DatetimeIndex(['20171111', '20181212']).repeat(2)

From 442f8882528f248113dea85a4e1a3102d7b0a41a Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Fri, 31 Aug 2018 00:28:24 -0700
Subject: [PATCH 03/11] Add test for Nat

---
 doc/source/whatsnew/v0.24.0.txt                  | 4 +++-
 pandas/tests/indexes/datetimes/test_timezones.py | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 3de38b2b3dcf9..80e583f86e19d 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -184,7 +184,7 @@ Other Enhancements
 - :class:`DatetimeIndex` gained :attr:`DatetimeIndex.timetz` attribute. Returns local time with timezone information. (:issue:`21358`)
 - :class:`Resampler` now is iterable like :class:`GroupBy` (:issue:`15314`).
 - :ref:`Series.resample` and :ref:`DataFrame.resample` have gained the :meth:`Resampler.quantile` (:issue:`15023`).
-- :class:`DatetimeIndex` has gained a `is_dst` method.
+- :class:`DatetimeIndex` has gained a ``is_dst`` method (:issue:`18885`, :issue:`18946`)
 
 .. _whatsnew_0240.api_breaking:
 
@@ -616,6 +616,8 @@ Timezones
 - Bug when setting a new value with :meth:`DataFrame.loc` with a :class:`DatetimeIndex` with a DST transition (:issue:`18308`, :issue:`20724`)
 - Bug in :meth:`DatetimeIndex.unique` that did not re-localize tz-aware dates correctly (:issue:`21737`)
 - Bug when indexing a :class:`Series` with a DST transition (:issue:`21846`)
+- Bug in :meth:`DatetimeIndex.floor` that raised an ``AmbiguousTimeError`` during a DST transition (:issue:`18946`)
+- Bug in :func:`merge` when merging ``datetime64[ns, tz]`` data that contained a DST transition (:issue:`18885`)
 
 Offsets
 ^^^^^^^
diff --git a/pandas/tests/indexes/datetimes/test_timezones.py b/pandas/tests/indexes/datetimes/test_timezones.py
index 55eb0468d7401..6af7a2362093c 100644
--- a/pandas/tests/indexes/datetimes/test_timezones.py
+++ b/pandas/tests/indexes/datetimes/test_timezones.py
@@ -1023,6 +1023,11 @@ def test_is_dst(self):
         expected = Index([False] * 4)
         tm.assert_index_equal(result, expected)
 
+        dti_nat = pd.DatetimeIndex([pd.NaT])
+        result = dti_nat.is_dst()
+        expected = Index([False])
+        tm.assert_index_equal(result, expected)
+
 
 class TestDateRange(object):
     """Tests for date_range with timezones"""

From 254b5a2b36b35bad4c744868157663598bc41153 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Fri, 31 Aug 2018 14:28:28 -0700
Subject: [PATCH 04/11] Add is_dst for timestamps

---
 doc/source/api.rst                              |  1 +
 doc/source/whatsnew/v0.24.0.txt                 |  2 +-
 pandas/_libs/tslibs/timestamps.pyx              | 14 ++++++++++++++
 pandas/tests/scalar/test_nat.py                 |  4 ++++
 pandas/tests/scalar/timestamp/test_timezones.py | 11 +++++++++++
 5 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/doc/source/api.rst b/doc/source/api.rst
index 1b299d03f13f6..ded4e1e3c55e9 100644
--- a/doc/source/api.rst
+++ b/doc/source/api.rst
@@ -1987,6 +1987,7 @@ Methods
     Timestamp.isocalendar
     Timestamp.isoformat
     Timestamp.isoweekday
+    Timestamp.is_dst
     Timestamp.month_name
     Timestamp.normalize
     Timestamp.now
diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 80e583f86e19d..e2109618b5ccd 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -184,7 +184,7 @@ Other Enhancements
 - :class:`DatetimeIndex` gained :attr:`DatetimeIndex.timetz` attribute. Returns local time with timezone information. (:issue:`21358`)
 - :class:`Resampler` now is iterable like :class:`GroupBy` (:issue:`15314`).
 - :ref:`Series.resample` and :ref:`DataFrame.resample` have gained the :meth:`Resampler.quantile` (:issue:`15023`).
-- :class:`DatetimeIndex` has gained a ``is_dst`` method (:issue:`18885`, :issue:`18946`)
+- :class:`DatetimeIndex` and :class:`Timestamp` have gained a ``is_dst`` method (:issue:`18885`, :issue:`18946`)
 
 .. _whatsnew_0240.api_breaking:
 
diff --git a/pandas/_libs/tslibs/timestamps.pyx b/pandas/_libs/tslibs/timestamps.pyx
index 3ab1396c0fe38..c06288f6cf23c 100644
--- a/pandas/_libs/tslibs/timestamps.pyx
+++ b/pandas/_libs/tslibs/timestamps.pyx
@@ -722,6 +722,20 @@ class Timestamp(_Timestamp):
         raise AttributeError("Cannot directly set timezone. Use tz_localize() "
                              "or tz_convert() as appropriate")
 
+    def is_dst(self):
+        """
+        Returns a boolean indicating if the Timestamp is in daylight savings
+        time. Naive timestamps are considered not to be in daylight savings
+        time.
+
+        Returns
+        -------
+        Boolean
+            True if the Timestamp is in daylight savings time
+            False if the Timestamp is naive or not in daylight savings time
+        """
+        return bool(self.dst())
+
     def __setstate__(self, state):
         self.value = state[0]
         self.freq = state[1]
diff --git a/pandas/tests/scalar/test_nat.py b/pandas/tests/scalar/test_nat.py
index a6b217a37bd0c..495532e778131 100644
--- a/pandas/tests/scalar/test_nat.py
+++ b/pandas/tests/scalar/test_nat.py
@@ -330,3 +330,7 @@ def test_nat_arithmetic_td64_vector(box, assert_func):
 def test_nat_pinned_docstrings():
     # GH17327
     assert NaT.ctime.__doc__ == datetime.ctime.__doc__
+
+
+def test_is_dst():
+    assert NaT.is_dst() is False
diff --git a/pandas/tests/scalar/timestamp/test_timezones.py b/pandas/tests/scalar/timestamp/test_timezones.py
index 8cebfafeae82a..7e7d1ee60856c 100644
--- a/pandas/tests/scalar/timestamp/test_timezones.py
+++ b/pandas/tests/scalar/timestamp/test_timezones.py
@@ -307,3 +307,14 @@ def test_timestamp_timetz_equivalent_with_datetime_tz(self,
         expected = _datetime.timetz()
 
         assert result == expected
+
+    def test_timestamp_is_dst(self):
+        ts_naive = Timestamp('2018-11-04')
+        assert ts_naive.is_dst() is False
+
+        ts_aware = ts_naive.tz_localize('US/Pacific')
+        assert ts_aware.is_dst() is True
+
+        # DST transition at 2am
+        ts_aware = Timestamp('2018-11-04 04:00').tz_localize('US/Pacific')
+        assert ts_aware.is_dst() is False

From 7c73d0a6ea5287130f114aaee5039d244c5d1013 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Fri, 31 Aug 2018 17:01:07 -0700
Subject: [PATCH 05/11] Add cython version of dst

---
 doc/source/whatsnew/v0.24.0.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index e2109618b5ccd..8c7d5636247bb 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -184,7 +184,7 @@ Other Enhancements
 - :class:`DatetimeIndex` gained :attr:`DatetimeIndex.timetz` attribute. Returns local time with timezone information. (:issue:`21358`)
 - :class:`Resampler` now is iterable like :class:`GroupBy` (:issue:`15314`).
 - :ref:`Series.resample` and :ref:`DataFrame.resample` have gained the :meth:`Resampler.quantile` (:issue:`15023`).
-- :class:`DatetimeIndex` and :class:`Timestamp` have gained a ``is_dst`` method (:issue:`18885`, :issue:`18946`)
+- :class:`DatetimeIndex` and :class:`Timestamp` have gained an ``is_dst`` method (:issue:`18885`, :issue:`18946`)
 
 .. _whatsnew_0240.api_breaking:
 

From b0938d5e92a97da93e3d7e0210b6775ed7292160 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Fri, 31 Aug 2018 18:25:54 -0700
Subject: [PATCH 06/11] Add cython version of is_dst

---
 pandas/_libs/tslib.pyx             |  2 +-
 pandas/_libs/tslibs/conversion.pyx | 13 +++--
 pandas/_libs/tslibs/period.pyx     |  2 +-
 pandas/_libs/tslibs/resolution.pyx |  2 +-
 pandas/_libs/tslibs/timezones.pxd  |  2 +-
 pandas/_libs/tslibs/timezones.pyx  | 76 +++++++++++++++++++++++++++---
 pandas/core/indexes/datetimes.py   |  4 +-
 7 files changed, 81 insertions(+), 20 deletions(-)

diff --git a/pandas/_libs/tslib.pyx b/pandas/_libs/tslib.pyx
index bdd279b19208b..fc53b88fd4cfe 100644
--- a/pandas/_libs/tslib.pyx
+++ b/pandas/_libs/tslib.pyx
@@ -146,7 +146,7 @@ def ints_to_pydatetime(int64_t[:] arr, tz=None, freq=None, box="datetime"):
                 dt64_to_dtstruct(local_value, &dts)
                 result[i] = func_create(value, dts, tz, freq)
     else:
-        trans, deltas, typ = get_dst_info(tz)
+        trans, deltas, typ = get_dst_info(tz, False)
 
         if typ not in ['pytz', 'dateutil']:
             # static/fixed; in this case we know that len(delta) == 1
diff --git a/pandas/_libs/tslibs/conversion.pyx b/pandas/_libs/tslibs/conversion.pyx
index fe664cf03b0b9..14939427c2da3 100644
--- a/pandas/_libs/tslibs/conversion.pyx
+++ b/pandas/_libs/tslibs/conversion.pyx
@@ -28,11 +28,10 @@ from np_datetime import OutOfBoundsDatetime
 
 from util cimport (is_string_object,
                    is_datetime64_object,
-                   is_integer_object, is_float_object, is_array)
+                   is_integer_object, is_float_object)
 
 from timedeltas cimport cast_from_unit
 from timezones cimport (is_utc, is_tzlocal, is_fixed_offset,
-                        treat_tz_as_dateutil, treat_tz_as_pytz,
                         get_utcoffset, get_dst_info,
                         get_timezone, maybe_get_tz, tz_compare)
 from parsing import parse_datetime_string
@@ -540,7 +539,7 @@ cdef inline void localize_tso(_TSObject obj, tzinfo tz):
         dt64_to_dtstruct(local_val, &obj.dts)
     else:
         # Adjust datetime64 timestamp, recompute datetimestruct
-        trans, deltas, typ = get_dst_info(tz)
+        trans, deltas, typ = get_dst_info(tz, False)
 
         if is_fixed_offset(tz):
             # static/fixed tzinfo; in this case we know len(deltas) == 1
@@ -636,7 +635,7 @@ cdef inline int64_t[:] _tz_convert_dst(int64_t[:] values, tzinfo tz,
         int64_t[:] deltas
         int64_t v
 
-    trans, deltas, typ = get_dst_info(tz)
+    trans, deltas, typ = get_dst_info(tz, False)
     if not to_utc:
         # We add `offset` below instead of subtracting it
         deltas = -1 * np.array(deltas, dtype='i8')
@@ -888,7 +887,7 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz, object ambiguous=None,
                              "the same size as vals")
         ambiguous_array = np.asarray(ambiguous)
 
-    trans, deltas, typ = get_dst_info(tz)
+    trans, deltas, typ = get_dst_info(tz, False)
 
     tdata = <int64_t*> cnp.PyArray_DATA(trans)
     ntrans = len(trans)
@@ -1150,7 +1149,7 @@ cdef int64_t[:] _normalize_local(int64_t[:] stamps, object tz):
             result[i] = _normalized_stamp(&dts)
     else:
         # Adjust datetime64 timestamp, recompute datetimestruct
-        trans, deltas, typ = get_dst_info(tz)
+        trans, deltas, typ = get_dst_info(tz, False)
 
         if typ not in ['pytz', 'dateutil']:
             # static/fixed; in this case we know that len(delta) == 1
@@ -1227,7 +1226,7 @@ def is_date_array_normalized(int64_t[:] stamps, tz=None):
             if (dts.hour + dts.min + dts.sec + dts.us) > 0:
                 return False
     else:
-        trans, deltas, typ = get_dst_info(tz)
+        trans, deltas, typ = get_dst_info(tz, False)
 
         if typ not in ['pytz', 'dateutil']:
             # static/fixed; in this case we know that len(delta) == 1
diff --git a/pandas/_libs/tslibs/period.pyx b/pandas/_libs/tslibs/period.pyx
index f68b6d8fdef57..6c29fd0280b02 100644
--- a/pandas/_libs/tslibs/period.pyx
+++ b/pandas/_libs/tslibs/period.pyx
@@ -1516,7 +1516,7 @@ cdef int64_t[:] localize_dt64arr_to_period(int64_t[:] stamps,
             result[i] = get_period_ordinal(&dts, freq)
     else:
         # Adjust datetime64 timestamp, recompute datetimestruct
-        trans, deltas, typ = get_dst_info(tz)
+        trans, deltas, typ = get_dst_info(tz, False)
 
         if typ not in ['pytz', 'dateutil']:
             # static/fixed; in this case we know that len(delta) == 1
diff --git a/pandas/_libs/tslibs/resolution.pyx b/pandas/_libs/tslibs/resolution.pyx
index 4e3350395400c..7ec3c0062c9e3 100644
--- a/pandas/_libs/tslibs/resolution.pyx
+++ b/pandas/_libs/tslibs/resolution.pyx
@@ -68,7 +68,7 @@ cdef _reso_local(int64_t[:] stamps, object tz):
                 reso = curr_reso
     else:
         # Adjust datetime64 timestamp, recompute datetimestruct
-        trans, deltas, typ = get_dst_info(tz)
+        trans, deltas, typ = get_dst_info(tz, False)
 
         if typ not in ['pytz', 'dateutil']:
             # static/fixed; in this case we know that len(delta) == 1
diff --git a/pandas/_libs/tslibs/timezones.pxd b/pandas/_libs/tslibs/timezones.pxd
index 8965b46f747c4..94eeaccf6284f 100644
--- a/pandas/_libs/tslibs/timezones.pxd
+++ b/pandas/_libs/tslibs/timezones.pxd
@@ -13,4 +13,4 @@ cpdef object maybe_get_tz(object tz)
 cdef get_utcoffset(tzinfo, obj)
 cdef bint is_fixed_offset(object tz)
 
-cdef object get_dst_info(object tz)
+cdef object get_dst_info(object tz, dst)
diff --git a/pandas/_libs/tslibs/timezones.pyx b/pandas/_libs/tslibs/timezones.pyx
index 36ec499c7335c..b200c6302ff38 100644
--- a/pandas/_libs/tslibs/timezones.pyx
+++ b/pandas/_libs/tslibs/timezones.pyx
@@ -186,16 +186,28 @@ cdef object get_utc_trans_times_from_dateutil_tz(object tz):
     return new_trans
 
 
-cdef int64_t[:] unbox_utcoffsets(object transinfo):
+cdef int64_t[:] unbox_utcoffsets(object transinfo, dst):
+    """
+    Unpack the offset information from pytz timezone objects
+
+    Parameters
+    ----------
+    transinfo : list of tuples
+        Each tuple contains (UTC offset, DST offset, tz abbreviation)
+    dst : boolean
+        True returns an array of the DST offsets
+        False returns an array of UTC offsets
+    """
     cdef:
         Py_ssize_t i, sz
         int64_t[:] arr
+        int key
 
     sz = len(transinfo)
     arr = np.empty(sz, dtype='i8')
-
+    key = int(dst)
     for i in range(sz):
-        arr[i] = int(transinfo[i][0].total_seconds()) * 1000000000
+        arr[i] = int(transinfo[i][key].total_seconds()) * 1000000000
 
     return arr
 
@@ -204,9 +216,22 @@ cdef int64_t[:] unbox_utcoffsets(object transinfo):
 # Daylight Savings
 
 
-cdef object get_dst_info(object tz):
+cdef object get_dst_info(object tz, dst):
     """
-    return a tuple of :
+    Return DST info from a timezone
+
+    Parameters
+    ----------
+    tz : object
+        timezone
+    dst : bool
+        True returns the DST specific offset
+        False returns the UTC offset
+        Specific for pytz timezones only
+
+    Returns
+    -------
+    tuple
       (UTC times of DST transitions,
        UTC offsets in microseconds corresponding to DST transitions,
        string of type of transitions)
@@ -230,7 +255,7 @@ cdef object get_dst_info(object tz):
                     trans[0] = NPY_NAT + 1
             except Exception:
                 pass
-            deltas = unbox_utcoffsets(tz._transition_info)
+            deltas = unbox_utcoffsets(tz._transition_info, dst)
             typ = 'pytz'
 
         elif treat_tz_as_dateutil(tz):
@@ -278,6 +303,45 @@ cdef object get_dst_info(object tz):
     return dst_cache[cache_key]
 
 
+def _is_dst(int64_t[:] values, object tz):
+    """
+    Return a boolean array where True indicates a value that lies in
+    daylight savings time and False indicates a value that does not lie in
+    daylight savings time
+
+    Parameters
+    ----------
+    values : ndarray
+        i8 representation of the datetimes
+    tz : object
+        timezone
+
+    Returns
+    -------
+    ndarray
+        Booleans
+    """
+    cdef:
+        Py_ssize_t n = len(values)
+        # Cython boolean memoryviews are not supported yet
+        # https://github.com/cython/cython/issues/2204
+        # bint[:] result
+        object typ
+
+    result = np.zeros(n, dtype=bool)
+    if tz is None:
+        return result
+    transitions, offsets, typ = get_dst_info(tz, True)
+    offsets = np.array(offsets)
+    # Fixed timezone offsets do not have DST transitions
+    if typ not in {'pytz', 'dateutil'}:
+        return result
+    positions = transitions.searchsorted(values, side='right')
+    # DST has 0 offset
+    result = offsets[positions] == 0
+    return result
+
+
 def infer_tzinfo(start, end):
     if start is not None and end is not None:
         tz = start.tzinfo
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 2358b1f2fb33d..e392d6222f975 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -471,9 +471,7 @@ def is_dst(self):
         >>> dti.is_dst()
         Index([True, True, False, False], dtype='object')
         """
-        if self.tz is None:
-            return Index([False] * len(self))
-        return Index([bool(ts.dst()) for ts in self])
+        return Index(timezones._is_dst(self.asi8, self.tz))
 
     @property
     def size(self):

From 641e29546b9d2f87a9bd433315dc59ae499021d1 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Fri, 31 Aug 2018 18:52:29 -0700
Subject: [PATCH 07/11] Add cython version of is_dst pt2

---
 pandas/_libs/tslibs/timezones.pyx                | 9 ++++-----
 pandas/tests/indexes/datetimes/test_timezones.py | 7 ++++++-
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/pandas/_libs/tslibs/timezones.pyx b/pandas/_libs/tslibs/timezones.pyx
index b200c6302ff38..f2f11898d042e 100644
--- a/pandas/_libs/tslibs/timezones.pyx
+++ b/pandas/_libs/tslibs/timezones.pyx
@@ -305,9 +305,8 @@ cdef object get_dst_info(object tz, dst):
 
 def _is_dst(int64_t[:] values, object tz):
     """
-    Return a boolean array where True indicates a value that lies in
-    daylight savings time and False indicates a value that does not lie in
-    daylight savings time
+    Return a boolean array indicating whether each epoch timestamp is in
+    daylight savings time with respect with the passed timezone.
 
     Parameters
     ----------
@@ -318,8 +317,8 @@ def _is_dst(int64_t[:] values, object tz):
 
     Returns
     -------
-    ndarray
-        Booleans
+    ndarray of booleans
+        True indicates daylight savings time
     """
     cdef:
         Py_ssize_t n = len(values)
diff --git a/pandas/tests/indexes/datetimes/test_timezones.py b/pandas/tests/indexes/datetimes/test_timezones.py
index 6af7a2362093c..179628f94fd12 100644
--- a/pandas/tests/indexes/datetimes/test_timezones.py
+++ b/pandas/tests/indexes/datetimes/test_timezones.py
@@ -1013,7 +1013,12 @@ def test_iteration_preserves_nanoseconds(self, tz):
             assert ts == index[i]
 
     def test_is_dst(self):
-        dti = pd.date_range('2018-11-04', periods=4, freq='H', tz='US/Pacific')
+        dti = DatetimeIndex([])
+        result = dti.is_dst()
+        expected = Index([])
+        tm.assert_index_equal(result, expected)
+
+        dti = date_range('2018-11-04', periods=4, freq='H', tz='US/Pacific')
         result = dti.is_dst()
         expected = Index([True, True, False, False])
         tm.assert_index_equal(result, expected)

From 820df35fba0948985aa501e11cbb8b8965ad17e8 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Fri, 31 Aug 2018 19:01:18 -0700
Subject: [PATCH 08/11] Add fixedoffset test

---
 pandas/tests/indexes/datetimes/test_timezones.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/pandas/tests/indexes/datetimes/test_timezones.py b/pandas/tests/indexes/datetimes/test_timezones.py
index 179628f94fd12..ee4cfb0f90719 100644
--- a/pandas/tests/indexes/datetimes/test_timezones.py
+++ b/pandas/tests/indexes/datetimes/test_timezones.py
@@ -1028,6 +1028,11 @@ def test_is_dst(self):
         expected = Index([False] * 4)
         tm.assert_index_equal(result, expected)
 
+        dti_fixed = dti.tz_localize(pytz.FixedOffset(300))
+        result = dti_fixed.is_dst()
+        expected = Index([False] * 4)
+        tm.assert_index_equal(result, expected)
+
         dti_nat = pd.DatetimeIndex([pd.NaT])
         result = dti_nat.is_dst()
         expected = Index([False])

From 9304b50eff22e9d243baaa36310fc2829a2ab570 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Mon, 3 Sep 2018 17:22:58 -0700
Subject: [PATCH 09/11] address review and test failures

---
 pandas/_libs/tslibs/nattype.pyx               | 15 ++++++++
 pandas/_libs/tslibs/timezones.pyx             | 21 +++++------
 pandas/core/indexes/datetimes.py              |  2 +-
 .../tests/indexes/datetimes/test_timezones.py | 36 +++++++------------
 4 files changed, 40 insertions(+), 34 deletions(-)

diff --git a/pandas/_libs/tslibs/nattype.pyx b/pandas/_libs/tslibs/nattype.pyx
index c0263b747b081..e05572f66525d 100644
--- a/pandas/_libs/tslibs/nattype.pyx
+++ b/pandas/_libs/tslibs/nattype.pyx
@@ -260,6 +260,20 @@ class NaTType(_NaT):
     def is_year_end(self):
         return False
 
+    def is_dst(self):
+        """
+        Returns a boolean indicating if the Timestamp is in daylight savings
+        time. Naive timestamps are considered not to be in daylight savings
+        time.
+
+        Returns
+        -------
+        Boolean
+            True if the Timestamp is in daylight savings time
+            False if the Timestamp is naive or not in daylight savings time
+        """
+        return False
+
     def __rdiv__(self, other):
         return _nat_rdivide_op(self, other)
 
@@ -353,6 +367,7 @@ class NaTType(_NaT):
     strptime = _make_error_func('strptime', datetime)
     strftime = _make_error_func('strftime', datetime)
     isocalendar = _make_error_func('isocalendar', datetime)
+    dst = _make_error_func('dst', datetime)
     ctime = _make_error_func('ctime', datetime)
     time = _make_error_func('time', datetime)
     toordinal = _make_error_func('toordinal', datetime)
diff --git a/pandas/_libs/tslibs/timezones.pyx b/pandas/_libs/tslibs/timezones.pyx
index f2f11898d042e..7c1fbf52553dc 100644
--- a/pandas/_libs/tslibs/timezones.pyx
+++ b/pandas/_libs/tslibs/timezones.pyx
@@ -108,7 +108,8 @@ def _p_tz_cache_key(tz):
     return tz_cache_key(tz)
 
 
-# Timezone data caches, key is the pytz string or dateutil file name.
+# Timezone data (UTC offset) caches
+# key is the pytz string or dateutil file name.
 dst_cache = {}
 
 
@@ -225,7 +226,8 @@ cdef object get_dst_info(object tz, dst):
     tz : object
         timezone
     dst : bool
-        True returns the DST specific offset
+        True returns the DST specific offset and will NOT store the results in
+        dst_cache. dst_cache is reserved for caching UTC offsets.
         False returns the UTC offset
         Specific for pytz timezones only
 
@@ -246,7 +248,7 @@ cdef object get_dst_info(object tz, dst):
                 np.array([num], dtype=np.int64),
                 None)
 
-    if cache_key not in dst_cache:
+    if cache_key not in dst_cache or dst:
         if treat_tz_as_pytz(tz):
             trans = np.array(tz._utc_transition_times, dtype='M8[ns]')
             trans = trans.view('i8')
@@ -298,12 +300,14 @@ cdef object get_dst_info(object tz, dst):
             deltas = np.array([num], dtype=np.int64)
             typ = 'static'
 
+        if dst:
+            return trans, deltas, typ
         dst_cache[cache_key] = (trans, deltas, typ)
 
     return dst_cache[cache_key]
 
 
-def _is_dst(int64_t[:] values, object tz):
+def is_dst(int64_t[:] values, object tz):
     """
     Return a boolean array indicating whether each epoch timestamp is in
     daylight savings time with respect with the passed timezone.
@@ -322,9 +326,6 @@ def _is_dst(int64_t[:] values, object tz):
     """
     cdef:
         Py_ssize_t n = len(values)
-        # Cython boolean memoryviews are not supported yet
-        # https://github.com/cython/cython/issues/2204
-        # bint[:] result
         object typ
 
     result = np.zeros(n, dtype=bool)
@@ -335,9 +336,9 @@ def _is_dst(int64_t[:] values, object tz):
     # Fixed timezone offsets do not have DST transitions
     if typ not in {'pytz', 'dateutil'}:
         return result
-    positions = transitions.searchsorted(values, side='right')
-    # DST has 0 offset
-    result = offsets[positions] == 0
+    positions = transitions.searchsorted(values, side='right') - 1
+    # DST has nonzero offset
+    result = offsets[positions] != 0
     return result
 
 
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index e392d6222f975..50dee697b39fb 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -471,7 +471,7 @@ def is_dst(self):
         >>> dti.is_dst()
         Index([True, True, False, False], dtype='object')
         """
-        return Index(timezones._is_dst(self.asi8, self.tz))
+        return Index(timezones.is_dst(self.asi8, self.tz))
 
     @property
     def size(self):
diff --git a/pandas/tests/indexes/datetimes/test_timezones.py b/pandas/tests/indexes/datetimes/test_timezones.py
index ee4cfb0f90719..2f5f4650ba948 100644
--- a/pandas/tests/indexes/datetimes/test_timezones.py
+++ b/pandas/tests/indexes/datetimes/test_timezones.py
@@ -1012,30 +1012,20 @@ def test_iteration_preserves_nanoseconds(self, tz):
         for i, ts in enumerate(index):
             assert ts == index[i]
 
-    def test_is_dst(self):
-        dti = DatetimeIndex([])
-        result = dti.is_dst()
-        expected = Index([])
-        tm.assert_index_equal(result, expected)
-
-        dti = date_range('2018-11-04', periods=4, freq='H', tz='US/Pacific')
+    @pytest.mark.parametrize('arg, expected_arg', [
+        [[], []],
+        [date_range('2018-11-04', periods=4, freq='H', tz='US/Pacific'),
+         [True, True, False, False]],
+        [date_range('2018-11-04', periods=4, freq='H'),
+         [False] * 4],
+        [date_range('2018-11-04', periods=4, freq='H', tz=pytz.FixedOffset(3)),
+         [False] * 4],
+        [[pd.NaT], [False]]
+    ])
+    def test_is_dst(self, arg, expected_arg):
+        dti = DatetimeIndex(arg)
         result = dti.is_dst()
-        expected = Index([True, True, False, False])
-        tm.assert_index_equal(result, expected)
-
-        dti_naive = dti.tz_localize(None)
-        result = dti_naive.is_dst()
-        expected = Index([False] * 4)
-        tm.assert_index_equal(result, expected)
-
-        dti_fixed = dti.tz_localize(pytz.FixedOffset(300))
-        result = dti_fixed.is_dst()
-        expected = Index([False] * 4)
-        tm.assert_index_equal(result, expected)
-
-        dti_nat = pd.DatetimeIndex([pd.NaT])
-        result = dti_nat.is_dst()
-        expected = Index([False])
+        expected = Index(expected_arg)
         tm.assert_index_equal(result, expected)
 
 

From 627bb19150fe2bdb0533f236476746c3e4426215 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Tue, 4 Sep 2018 19:11:58 -0700
Subject: [PATCH 10/11] type dst and document

---
 pandas/_libs/tslibs/timezones.pxd |  2 +-
 pandas/_libs/tslibs/timezones.pyx | 18 +++++++++++-------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/pandas/_libs/tslibs/timezones.pxd b/pandas/_libs/tslibs/timezones.pxd
index 94eeaccf6284f..638bd0e79c806 100644
--- a/pandas/_libs/tslibs/timezones.pxd
+++ b/pandas/_libs/tslibs/timezones.pxd
@@ -13,4 +13,4 @@ cpdef object maybe_get_tz(object tz)
 cdef get_utcoffset(tzinfo, obj)
 cdef bint is_fixed_offset(object tz)
 
-cdef object get_dst_info(object tz, dst)
+cdef object get_dst_info(object tz, bint dst)
diff --git a/pandas/_libs/tslibs/timezones.pyx b/pandas/_libs/tslibs/timezones.pyx
index 7c1fbf52553dc..e32cab36a4e82 100644
--- a/pandas/_libs/tslibs/timezones.pyx
+++ b/pandas/_libs/tslibs/timezones.pyx
@@ -187,9 +187,10 @@ cdef object get_utc_trans_times_from_dateutil_tz(object tz):
     return new_trans
 
 
-cdef int64_t[:] unbox_utcoffsets(object transinfo, dst):
+cdef int64_t[:] unbox_utcoffsets(object transinfo, bint dst):
     """
-    Unpack the offset information from pytz timezone objects
+    Unpack the offset information from the _transition_info attribute of pytz
+    timezones
 
     Parameters
     ----------
@@ -206,9 +207,10 @@ cdef int64_t[:] unbox_utcoffsets(object transinfo, dst):
 
     sz = len(transinfo)
     arr = np.empty(sz, dtype='i8')
-    key = int(dst)
     for i in range(sz):
-        arr[i] = int(transinfo[i][key].total_seconds()) * 1000000000
+        # If dst == True, extract the DST shift in nanoseconds
+        # If dst == False, extract the UTC offset in nanoseconds
+        arr[i] = int(transinfo[i][dst].total_seconds()) * 1000000000
 
     return arr
 
@@ -217,15 +219,15 @@ cdef int64_t[:] unbox_utcoffsets(object transinfo, dst):
 # Daylight Savings
 
 
-cdef object get_dst_info(object tz, dst):
+cdef object get_dst_info(object tz, bint dst):
     """
     Return DST info from a timezone
 
     Parameters
     ----------
     tz : object
-        timezone
-    dst : bool
+        timezone object
+    dst : bint
         True returns the DST specific offset and will NOT store the results in
         dst_cache. dst_cache is reserved for caching UTC offsets.
         False returns the UTC offset
@@ -333,10 +335,12 @@ def is_dst(int64_t[:] values, object tz):
         return result
     transitions, offsets, typ = get_dst_info(tz, True)
     offsets = np.array(offsets)
+
     # Fixed timezone offsets do not have DST transitions
     if typ not in {'pytz', 'dateutil'}:
         return result
     positions = transitions.searchsorted(values, side='right') - 1
+
     # DST has nonzero offset
     result = offsets[positions] != 0
     return result

From c03792e22c10a1affdfde6c119c728b4451f1434 Mon Sep 17 00:00:00 2001
From: Matt Roeschke <mroeschke@housecanary.com>
Date: Tue, 4 Sep 2018 19:20:11 -0700
Subject: [PATCH 11/11] Add dateutil test for scalars

---
 pandas/tests/scalar/timestamp/test_timezones.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/scalar/timestamp/test_timezones.py b/pandas/tests/scalar/timestamp/test_timezones.py
index 7e7d1ee60856c..54977cc7bfbf8 100644
--- a/pandas/tests/scalar/timestamp/test_timezones.py
+++ b/pandas/tests/scalar/timestamp/test_timezones.py
@@ -308,13 +308,14 @@ def test_timestamp_timetz_equivalent_with_datetime_tz(self,
 
         assert result == expected
 
-    def test_timestamp_is_dst(self):
+    @pytest.mark.parametrize('tz', ['US/Pacific', 'dateutil/US/Pacific'])
+    def test_timestamp_is_dst(self, tz):
         ts_naive = Timestamp('2018-11-04')
         assert ts_naive.is_dst() is False
 
-        ts_aware = ts_naive.tz_localize('US/Pacific')
+        ts_aware = ts_naive.tz_localize(tz)
         assert ts_aware.is_dst() is True
 
         # DST transition at 2am
-        ts_aware = Timestamp('2018-11-04 04:00').tz_localize('US/Pacific')
+        ts_aware = Timestamp('2018-11-04 04:00').tz_localize(tz)
         assert ts_aware.is_dst() is False