TST: Additional to iloc setitem

This tests make sure when converting multiple columns to datetimes
and when assiging back it remains as datetime not as unix date
as mentioned in GH #20511.

Author: parthi-siva

pandas/tests/indexing/test_iloc.py

@@ -21,11 +21,13 @@
     Interval,
     NaT,
     Series,
+    Timestamp,
     array,
     concat,
     date_range,
     interval_range,
     isna,
+    to_datetime,
 )
 import pandas._testing as tm
 from pandas.api.types import is_scalar
@@ -1188,6 +1190,24 @@ def test_iloc_getitem_int_single_ea_block_view(self):
         arr[2] = arr[-1]
         assert ser[0] == arr[-1]
 
+    def test_iloc_setitem_multicolumn_to_datetime(self):
+
+        # GH#20511
+        df = DataFrame({"A": ["2022-01-01", "2022-01-02"], "B": ["2021", "2022"]})
+
+        df.iloc[:, [0]] = DataFrame({"A": to_datetime(["2021", "2022"])})
+        expected = DataFrame(
+            {
+                "A": [
+                    Timestamp("2021-01-01 00:00:00"),
+                    Timestamp("2022-01-01 00:00:00"),
+                ],
+                "B": ["2021", "2022"],
+            },
+            dtype=object,
+        )
+        tm.assert_frame_equal(df, expected)
+
 
 class TestILocErrors:
     # NB: this test should work for _any_ Series we can pass as
@@ -1363,6 +1383,24 @@ def test_frame_iloc_setitem_callable(self):
         exp.iloc[[1, 3], [0]] = [-5, -5]
         tm.assert_frame_equal(res, exp)
 
+    def test_frame_iloc_setitem_callable_multicolumn_to_datetime(self):
+
+        # GH#20511
+        df = DataFrame({"A": ["2022-01-01", "2022-01-02"], "B": ["2021", "2022"]})
+
+        df.iloc[:, [0]] = df.iloc[:, [0]].apply(to_datetime)
+        expected = DataFrame(
+            {
+                "A": [
+                    Timestamp("2022-01-01 00:00:00"),
+                    Timestamp("2022-01-02 00:00:00"),
+                ],
+                "B": ["2021", "2022"],
+            },
+            dtype=object,
+        )
+        tm.assert_frame_equal(df, expected)
+
 
 class TestILocSeries:
     def test_iloc(self):