Fix cudf.Scalar string datetime construction (#9875)

Closes #9874

Authors:
  - https://github.com/brandon-b-miller

Approvers:
  - Michael Wang (https://github.com/isVoid)

URL: #9875

python/cudf/cudf/tests/test_scalar.py

@@ -369,3 +369,15 @@ def test_construct_from_scalar(value):
 
     x._is_host_value_current == y._is_host_value_current
     x._is_device_value_current == y._is_device_value_current
+
+
+@pytest.mark.parametrize(
+    "data", ["20000101", "2000-01-01", "2000-01-01T00:00:00.000000000", "2000"]
+)
+@pytest.mark.parametrize("dtype", DATETIME_TYPES)
+def test_datetime_scalar_from_string(data, dtype):
+    slr = cudf.Scalar(data, dtype)
+
+    expected = np.datetime64(datetime.datetime(2000, 1, 1)).astype(dtype)
+
+    assert expected == slr.value

python/cudf/cudf/utils/dtypes.py

@@ -257,7 +257,11 @@ def to_cudf_compatible_scalar(val, dtype=None):
     val = cudf.api.types.pandas_dtype(type(val)).type(val)
 
     if dtype is not None:
-        val = val.astype(dtype)
+        if isinstance(val, str) and np.dtype(dtype).kind == "M":
+            # pd.Timestamp can handle str, but not np.str_
+            val = pd.Timestamp(str(val)).to_datetime64().astype(dtype)
+        else:
+            val = val.astype(dtype)
 
     if val.dtype.type is np.datetime64:
         time_unit, _ = np.datetime_data(val.dtype)