BUG: in Timestamp.replace when replacing tzinfo around DST changes

closes pandas-dev#15683

Author: Stefan Scherfke

doc/source/whatsnew/v0.21.0.txt

@@ -487,6 +487,7 @@ Conversion
 - Bug in ``IntervalIndex.is_non_overlapping_monotonic`` when intervals are closed on both sides and overlap at a point (:issue:`16560`)
 - Bug in :func:`Series.fillna` returns frame when ``inplace=True`` and ``value`` is dict (:issue:`16156`)
 - Bug in :attr:`Timestamp.weekday_name` returning a UTC-based weekday name when localized to a timezone (:issue:`17354`)
+- Bug in ``Timestamp.replace`` when replacing ``tzinfo`` around DST changes (:issue:`15683`)
 
 Indexing
 ^^^^^^^^

pandas/_libs/tslib.pyx

@@ -683,14 +683,16 @@ class Timestamp(_Timestamp):
 
         cdef:
             pandas_datetimestruct dts
-            int64_t value
-            object _tzinfo, result, k, v
+            int64_t value, value_tz, offset
+            object _tzinfo, result, k, v, ts_input
 
         # set to naive if needed
         _tzinfo = self.tzinfo
         value = self.value
         if _tzinfo is not None:
-            value = tz_convert_single(value, 'UTC', _tzinfo)
+            value_tz = tz_convert_single(value, _tzinfo, 'UTC')
+            offset = value - value_tz
+            value += offset
 
         # setup components
         pandas_datetime_to_datetimestruct(value, PANDAS_FR_ns, &dts)
@@ -724,16 +726,14 @@ class Timestamp(_Timestamp):
             _tzinfo = tzinfo
 
         # reconstruct & check bounds
-        value = pandas_datetimestruct_to_datetime(PANDAS_FR_ns, &dts)
+        ts_input = datetime(dts.year, dts.month, dts.day, dts.hour, dts.min,
+                            dts.sec, dts.us, tzinfo=_tzinfo)
+        ts = convert_to_tsobject(ts_input, _tzinfo, None, 0, 0)
+        value = ts.value + (dts.ps // 1000)
         if value != NPY_NAT:
             _check_dts_bounds(&dts)
 
-        # set tz if needed
-        if _tzinfo is not None:
-            value = tz_convert_single(value, _tzinfo, 'UTC')
-
-        result = create_timestamp_from_ts(value, dts, _tzinfo, self.freq)
-        return result
+        return create_timestamp_from_ts(value, dts, _tzinfo, self.freq)
 
     def isoformat(self, sep='T'):
         base = super(_Timestamp, self).isoformat(sep=sep)

pandas/tests/tseries/test_timezones.py

@@ -1269,6 +1269,27 @@ def test_ambiguous_compat(self):
             assert (result_pytz.to_pydatetime().tzname() ==
                     result_dateutil.to_pydatetime().tzname())
 
+    def test_replace_tzinfo(self):
+        # GH 15683
+        dt = datetime(2016, 3, 27, 1)
+        tzinfo = pytz.timezone('CET').localize(dt, is_dst=False).tzinfo
+
+        result_dt = dt.replace(tzinfo=tzinfo)
+        result_pd = Timestamp(dt).replace(tzinfo=tzinfo)
+
+        if hasattr(result_dt, 'timestamp'):  # New method in Py 3.3
+            assert result_dt.timestamp() == result_pd.timestamp()
+        assert result_dt == result_pd
+        assert result_dt == result_pd.to_pydatetime()
+
+        result_dt = dt.replace(tzinfo=tzinfo).replace(tzinfo=None)
+        result_pd = Timestamp(dt).replace(tzinfo=tzinfo).replace(tzinfo=None)
+
+        if hasattr(result_dt, 'timestamp'):  # New method in Py 3.3
+            assert result_dt.timestamp() == result_pd.timestamp()
+        assert result_dt == result_pd
+        assert result_dt == result_pd.to_pydatetime()
+
     def test_index_equals_with_tz(self):
         left = date_range('1/1/2011', periods=100, freq='H', tz='utc')
         right = date_range('1/1/2011', periods=100, freq='H', tz='US/Eastern')