handle DST appropriately in Timestamp.replace

doc/source/whatsnew/v0.22.0.txt

@@ -248,4 +248,4 @@ Other
 - Fixed a bug where creating a Series from an array that contains both tz-naive and tz-aware values will result in a Series whose dtype is tz-aware instead of object (:issue:`16406`)
 - Fixed construction of a :class:`Series` from a ``dict`` containing ``NaN`` as key (:issue:`18480`)
 - Adding a ``Period`` object to a ``datetime`` or ``Timestamp`` object will now correctly raise a ``TypeError`` (:issue:`17983`)
--
+- :func:`Timestamp.replace` will now handle Daylight Savings transitions gracefully (:issue:`18319`)

pandas/_libs/tslibs/timestamps.pyx

@@ -32,7 +32,7 @@ from np_datetime cimport (reverse_ops, cmp_scalar, check_dts_bounds,
                           is_leapyear)
 from timedeltas import Timedelta
 from timedeltas cimport delta_to_nanoseconds
-from timezones cimport get_timezone, is_utc, maybe_get_tz
+from timezones cimport get_timezone, is_utc, maybe_get_tz, treat_tz_as_pytz
 
 # ----------------------------------------------------------------------
 # Constants
@@ -927,8 +927,18 @@ class Timestamp(_Timestamp):
             _tzinfo = tzinfo
 
         # reconstruct & check bounds
-        ts_input = datetime(dts.year, dts.month, dts.day, dts.hour, dts.min,
-                            dts.sec, dts.us, tzinfo=_tzinfo)
+        if _tzinfo is not None and treat_tz_as_pytz(_tzinfo):
+            # replacing across a DST boundary may induce a new tzinfo object
+            # see GH#18319
+            ts_input = _tzinfo.localize(datetime(dts.year, dts.month, dts.day,
+                                                 dts.hour, dts.min, dts.sec,
+                                                 dts.us))
+            _tzinfo = ts_input.tzinfo
+        else:
+            ts_input = datetime(dts.year, dts.month, dts.day,
+                                dts.hour, dts.min, dts.sec, dts.us,
+                                tzinfo=_tzinfo)
+
         ts = convert_datetime_to_tsobject(ts_input, _tzinfo)
         value = ts.value + (dts.ps // 1000)
         if value != NPY_NAT:

pandas/tests/tseries/test_timezones.py

@@ -1229,6 +1229,26 @@ def f():
         dt = Timestamp('2013-11-03 01:59:59.999999-0400', tz='US/Eastern')
         assert dt.tz_localize(None) == dt.replace(tzinfo=None)
 
+    def test_replace_across_dst(self):
+        # GH#18319
+        tz = pytz.timezone('US/Eastern')
+
+        ts_naive = Timestamp('2017-12-03 16:03:30')
+        ts_aware = tz.localize(ts_naive)
+
+        # Preliminary sanity-check
+        assert ts_aware == ts_aware.tzinfo.normalize(ts_aware)
+
+        # Replace across DST boundary
+        ts2 = ts_aware.replace(month=6)
+
+        # Check that `replace` preserves hour literal
+        assert (ts2.hour, ts2.minute) == (ts_aware.hour, ts_aware.minute)
+
+        # Check that post-replace object is appropriately normalized
+        ts2b = ts2.tzinfo.normalize(ts2)
+        assert ts2 == ts2b
+
     def test_ambiguous_compat(self):
         # validate that pytz and dateutil are compat for dst
         # when the transition happens