TST: added test for df.loc modify datetime columns (pandas-dev#28964)

* TST: added test for df.loc modifies datetime columns

Issue number pandas-dev#28837

* ran black pandas command

* MAINT: Address reviewer comments

Author: rohitsanj

pandas/tests/indexing/test_loc.py

@@ -712,6 +712,32 @@ def test_loc_assign_non_ns_datetime(self, unit):
         expected = Series(df.loc[:, "expected"], name=unit)
         tm.assert_series_equal(df.loc[:, unit], expected)
 
+    def test_loc_modify_datetime(self):
+        # see gh-28837
+        df = DataFrame.from_dict(
+            {"date": [1485264372711, 1485265925110, 1540215845888, 1540282121025]}
+        )
+
+        df["date_dt"] = pd.to_datetime(df["date"], unit="ms", cache=True)
+
+        df.loc[:, "date_dt_cp"] = df.loc[:, "date_dt"]
+        df.loc[[2, 3], "date_dt_cp"] = df.loc[[2, 3], "date_dt"]
+
+        expected = DataFrame(
+            [
+                [1485264372711, "2017-01-24 13:26:12.711", "2017-01-24 13:26:12.711"],
+                [1485265925110, "2017-01-24 13:52:05.110", "2017-01-24 13:52:05.110"],
+                [1540215845888, "2018-10-22 13:44:05.888", "2018-10-22 13:44:05.888"],
+                [1540282121025, "2018-10-23 08:08:41.025", "2018-10-23 08:08:41.025"],
+            ],
+            columns=["date", "date_dt", "date_dt_cp"],
+        )
+
+        columns = ["date_dt", "date_dt_cp"]
+        expected[columns] = expected[columns].apply(pd.to_datetime)
+
+        tm.assert_frame_equal(df, expected)
+
     def test_loc_setitem_frame(self):
         df = self.frame_labels