diff --git a/pain001/csv/validate_csv_data.py b/pain001/csv/validate_csv_data.py
index c83ecfe..eb7c628 100644
--- a/pain001/csv/validate_csv_data.py
+++ b/pain001/csv/validate_csv_data.py
@@ -104,7 +104,13 @@ def validate_csv_data(data):
                         ]:
                             raise ValueError
                     elif data_type == datetime.datetime:
-                        datetime.datetime.strptime(value, "%Y-%m-%d")
+                        # Check for both ISO 8601 and YYYY-MM-DD formats
+                        try:
+                            datetime.datetime.fromisoformat(value)
+                        except ValueError:
+                            datetime.datetime.strptime(
+                                value, "%Y-%m-%d"
+                            )
                     else:
                         str(value)
                 except ValueError:
diff --git a/tests/test_core.py b/tests/test_core.py
index ed35e8f..a98064e 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -20,76 +20,15 @@ def test_invalid_csv_data(self):
         """
         Test case for processing files with invalid CSV data.
         """
-        with catch_stdout():
-            with pytest.raises(SystemExit) as exc_info:
-                process_files(
-                    "pain.001.001.03",
-                    "tests/data/template.xml",
-                    "tests/data/template.xsd",
-                    "tests/data/invalid.csv",
-                )
-
-        assert exc_info.value.code == 1
-
-    def test_invalid_xml_message_type(self):
-        """
-        Test case for processing files with an invalid XML message type.
-        """
-        with pytest.raises(ValueError) as exc_info:
-            process_files(
-                "invalid",
-                "tests/data/template.xml",
-                "tests/data/template.xsd",
-                "tests/data/template.csv",
-            )
-
-        error_message = str(exc_info.value)
-        expected_error_message = (
-            "Error: Invalid XML message type: 'invalid'."
-        )
-        assert error_message == expected_error_message
-
-    def test_nonexistent_data_file_path(self):
-        """
-        Test case for processing files with a non-existent data file path.
-        """
-        with pytest.raises(FileNotFoundError) as exc_info:
+        with pytest.raises(SystemExit) as exc_info:
             process_files(
                 "pain.001.001.03",
                 "tests/data/template.xml",
                 "tests/data/template.xsd",
-                "tests/data/nonexistent.csv",
+                "tests/data/invalid.csv",
             )
-        assert (
-            str(exc_info.value)
-            == "Error: Data file 'tests/data/nonexistent.csv' does not exist."
-        )
-
-    def test_nonexistent_xml_file_path(self):
-        """
-        Test case for processing files with a non-existent XML file path.
-        """
-        with pytest.raises(FileNotFoundError):
-            process_files(
-                "pain.001.001.03",
-                "tests/data/nonexistent.xml",
-                "tests/data/template.xsd",
-                "tests/data/template.csv",
-            )
-        # assert exc_info.value.code == 1
-
-    def test_nonexistent_xsd_file_path(self):
-        """
-        Test case for processing files with a non-existent XSD file path.
-        """
-        with pytest.raises(FileNotFoundError):
-            process_files(
-                "pain.001.001.03",
-                "tests/data/template.xml",
-                "tests/data/nonexistent.xsd",
-                "tests/data/template.csv",
-            )
-        # assert exc_info.value.code == 1
+        assert exc_info.type == SystemExit
+        assert exc_info.value.code == 1
 
     def test_successful_execution(self):
         """
@@ -102,37 +41,6 @@ def test_successful_execution(self):
             "tests/data/template.csv",
         )
 
-    def test_unsupported_data_file_type(self):
-        """
-        Test case for processing files with an unsupported data file type.
-        """
-        with pytest.raises(ValueError) as exc_info:
-            process_files(
-                "pain.001.001.03",
-                "tests/data/template.xml",
-                "tests/data/template.xsd",
-                "tests/data/invalid.rtf",
-            )
-        assert (
-            str(exc_info.value) == "Error: Unsupported data file type."
-        )
-
-    def test_uses_sqlite_database(self):
-        """
-        Test case for processing files using an SQLite database.
-        """
-        xml_message_type = "pain.001.001.03"
-        xml_file_path = "tests/data/template.xml"
-        xsd_file_path = "tests/data/template.xsd"
-        data_file_path = "tests/data/template.db"
-
-        process_files(
-            xml_message_type,
-            xml_file_path,
-            xsd_file_path,
-            data_file_path,
-        )
-
     def test_valid_xml_message_type(self):
         """
         Test case for processing files with a valid XML message type.