Metadata/data validation should produce a warning for an unused datetime_format entry

sdv/metadata/metadata.py

@@ -4,7 +4,7 @@
 
 import pandas as pd
 
-from sdv._utils import _validate_boolean_parameter
+from sdv._utils import _is_datetime_type, _is_numerical, _validate_boolean_parameter
 from sdv.metadata.errors import InvalidMetadataError
 from sdv.metadata.multi_table import MultiTableMetadata
 from sdv.metadata.single_table import SingleTableMetadata
@@ -308,6 +308,35 @@ def set_sequence_key(self, column_name, table_name=None):
         self._validate_table_exists(table_name)
         self.tables[table_name].set_sequence_key(column_name)
 
+    def _validate_data(self, data, table_name=None):
+        """Validate data and warn about datetime formats that cannot be verified."""
+        super()._validate_data(data, table_name)
+        for current_table_name, table_data in data.items():
+            table_metadata = self.tables.get(current_table_name)
+            if table_metadata is None:
+                continue
+
+            for column_name, column_metadata in table_metadata.columns.items():
+                datetime_format = column_metadata.get('datetime_format')
+                if not datetime_format:
+                    continue
+
+                column_data = table_data[column_name]
+                has_datetime_objects = any(
+                    not isinstance(value, str)
+                    and not _is_numerical(value)
+                    and _is_datetime_type(value)
+                    for value in column_data.dropna().head(1000)
+                )
+                if has_datetime_objects:
+                    warnings.warn(
+                        f"The datetime format for column '{column_name}' "
+                        f"(table '{current_table_name}') could not be verified because the data "
+                        f"is represented as dtype '{column_data.dtype}'.\n"
+                        'Please omit the datetime format string from the metadata or cast the '
+                        'data to strings with the right format.'
+                    )
+
     def validate_table(self, data, table_name=None):
         """Validate a table against the metadata.
 

sdv/single_table/base.py

@@ -651,6 +651,11 @@ def validate(self, data):
                     message=r"No 'datetime_format' is present.*",
                     category=UserWarning,
                 )
+                warnings.filterwarnings(
+                    'ignore',
+                    message=r'The datetime format for column .* could not be verified.*',
+                    category=UserWarning,
+                )
                 self._original_metadata.validate_data({self._table_name: data})
         else:
             self._original_metadata.validate_data({self._table_name: data})

sdv/utils/utils.py

@@ -64,6 +64,11 @@ def drop_unknown_references(data, metadata, drop_missing_values=False, verbose=T
                 message=r"No 'datetime_format' is present.*",
                 category=UserWarning,
             )
+            warnings.filterwarnings(
+                'ignore',
+                message=r'The datetime format for column .* could not be verified.*',
+                category=UserWarning,
+            )
             metadata.validate_data(data)
         if drop_missing_values:
             _validate_foreign_keys_not_null(metadata, data)

tests/unit/metadata/test_metadata.py

@@ -1,4 +1,5 @@
 import re
+from datetime import date, datetime
 from unittest.mock import Mock, call, mock_open, patch
 
 import pandas as pd
@@ -574,6 +575,74 @@ def test_validate_data_no_relationships(self):
         metadata.validate_data(data)
         assert metadata.METADATA_SPEC_VERSION == 'V1'
 
+    def test_validate_data_warns_when_datetime_format_cannot_be_verified(self):
+        """Test warnings for formatted datetime columns without verifiable formats."""
+        # Setup
+        metadata = Metadata.load_from_dict({
+            'tables': {
+                'guests': {
+                    'columns': {
+                        'datetime64_date': {
+                            'sdtype': 'datetime',
+                            'datetime_format': '%d %b %Y',
+                        },
+                        'timestamp_date': {
+                            'sdtype': 'datetime',
+                            'datetime_format': '%Y-%m-%d',
+                        },
+                        'datetime_date': {
+                            'sdtype': 'datetime',
+                            'datetime_format': '%Y-%m-%d',
+                        },
+                        'date_date': {
+                            'sdtype': 'datetime',
+                            'datetime_format': '%Y-%m-%d',
+                        },
+                        'string_date': {
+                            'sdtype': 'datetime',
+                            'datetime_format': '%d %b %Y',
+                        },
+                        'numerical_date': {
+                            'sdtype': 'datetime',
+                            'datetime_format': '%Y%m%d',
+                        },
+                    },
+                },
+            },
+        })
+        data = {
+            'guests': pd.DataFrame({
+                'datetime64_date': pd.to_datetime(['2026-07-21']),
+                'timestamp_date': pd.Series([pd.Timestamp('2026-07-21')], dtype='object'),
+                'datetime_date': pd.Series([datetime(2026, 7, 21)], dtype='object'),
+                'date_date': pd.Series([date(2026, 7, 21)], dtype='object'),
+                'string_date': ['21 Jul 2026'],
+                'numerical_date': [20260721],
+            }),
+        }
+
+        # Run
+        with pytest.warns(UserWarning) as raised_warnings:
+            metadata.validate_data(data)
+
+        # Assert
+        expected_warning_columns = {
+            'datetime64_date': 'datetime64[ns]',
+            'timestamp_date': 'object',
+            'datetime_date': 'object',
+            'date_date': 'object',
+        }
+        expected_messages = [
+            (
+                f"The datetime format for column '{column_name}' (table 'guests') could not be "
+                f"verified because the data is represented as dtype '{dtype}'.\n"
+                'Please omit the datetime format string from the metadata or cast the data to '
+                'strings with the right format.'
+            )
+            for column_name, dtype in expected_warning_columns.items()
+        ]
+        assert [str(warning.message) for warning in raised_warnings] == expected_messages
+
     def test_validate_table(self):
         """Test the ``validate_table``method."""
         # Setup