BUG: to_datetime(strs, utc=True) used previous UTC offset (#25020)

Author: mroeschke

doc/source/whatsnew/v0.25.0.rst

@@ -103,7 +103,7 @@ Timedelta
 Timezones
 ^^^^^^^^^
 
--
+- Bug in :func:`to_datetime` with ``utc=True`` and datetime strings that would apply previously parsed UTC offsets to subsequent arguments (:issue:`24992`)
 -
 -
 

pandas/_libs/tslib.pyx

@@ -645,6 +645,8 @@ cpdef array_to_datetime(ndarray[object] values, str errors='raise',
                             out_tzoffset_vals.add(out_tzoffset * 60.)
                             tz = pytz.FixedOffset(out_tzoffset)
                             value = tz_convert_single(value, tz, UTC)
+                            out_local = 0
+                            out_tzoffset = 0
                         else:
                             # Add a marker for naive string, to track if we are
                             # parsing mixed naive and aware strings

pandas/tests/indexes/datetimes/test_tools.py

@@ -714,6 +714,29 @@ def test_iso_8601_strings_with_different_offsets(self):
                                   NaT], tz='UTC')
         tm.assert_index_equal(result, expected)
 
+    def test_iss8601_strings_mixed_offsets_with_naive(self):
+        # GH 24992
+        result = pd.to_datetime([
+            '2018-11-28T00:00:00',
+            '2018-11-28T00:00:00+12:00',
+            '2018-11-28T00:00:00',
+            '2018-11-28T00:00:00+06:00',
+            '2018-11-28T00:00:00'
+        ], utc=True)
+        expected = pd.to_datetime([
+            '2018-11-28T00:00:00',
+            '2018-11-27T12:00:00',
+            '2018-11-28T00:00:00',
+            '2018-11-27T18:00:00',
+            '2018-11-28T00:00:00'
+        ], utc=True)
+        tm.assert_index_equal(result, expected)
+
+        items = ['2018-11-28T00:00:00+12:00', '2018-11-28T00:00:00']
+        result = pd.to_datetime(items, utc=True)
+        expected = pd.to_datetime(list(reversed(items)), utc=True)[::-1]
+        tm.assert_index_equal(result, expected)
+
     def test_non_iso_strings_with_tz_offset(self):
         result = to_datetime(['March 1, 2018 12:00:00+0400'] * 2)
         expected = DatetimeIndex([datetime(2018, 3, 1, 12,