BUG: Return UTC DateTimeIndex when specified in to_datetime

Title is self-explanatory.  Closes pandas-dev#11934.

Author: gfyoung <[email protected]>

Closes pandas-dev#12391 from gfyoung/to_datetime_utc and squashes the following commits:

fa794a5 [gfyoung] BUG: Return UTC DateTimeIndex when specified in to_datetime

Author: gfyoung

doc/source/whatsnew/v0.18.0.txt

@@ -1108,3 +1108,4 @@ Bug Fixes
 
 - Bug in ``DataFrame.apply`` in which reduction was not being prevented for cases in which ``dtype`` was not a numpy dtype (:issue:`12244`)
 - Bug when initializing categorical series with a scalar value. (:issue:`12336`)
+- Bug when specifying a UTC ``DatetimeIndex`` by setting ``utc=True`` in ``.to_datetime`` (:issue:11934)

pandas/tseries/tests/test_timeseries.py

@@ -1143,6 +1143,16 @@ def test_to_datetime_tz_pytz(self):
                                  dtype='datetime64[ns, UTC]', freq=None)
         tm.assert_index_equal(result, expected)
 
+    def test_to_datetime_utc_is_true(self):
+        # See gh-11934
+        start = pd.Timestamp('2014-01-01', tz='utc')
+        end = pd.Timestamp('2014-01-03', tz='utc')
+        date_range = pd.bdate_range(start, end)
+
+        result = pd.to_datetime(date_range, utc=True)
+        expected = pd.DatetimeIndex(data=date_range)
+        tm.assert_index_equal(result, expected)
+
     def test_to_datetime_tz_psycopg2(self):
 
         # xref 8260
@@ -1175,7 +1185,8 @@ def test_to_datetime_tz_psycopg2(self):
         tm.assert_index_equal(result, i)
 
         result = pd.to_datetime(i, errors='coerce', utc=True)
-        expected = pd.DatetimeIndex(['2000-01-01 13:00:00'])
+        expected = pd.DatetimeIndex(['2000-01-01 13:00:00'],
+                                    dtype='datetime64[ns, UTC]')
         tm.assert_index_equal(result, expected)
 
     def test_index_to_datetime(self):

pandas/tseries/tools.py

@@ -310,7 +310,7 @@ def _convert_listlike(arg, box, format, name=None):
             if not isinstance(arg, DatetimeIndex):
                 return DatetimeIndex(arg, tz='utc' if utc else None)
             if utc:
-                arg = arg.tz_convert(None)
+                arg = arg.tz_convert(None).tz_localize('UTC')
             return arg
 
         elif format is None and com.is_integer_dtype(arg) and unit == 'ns':