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

[npatki]

Problem Description

For datetime columns, the metadata specification allows me to input a datetime_format string that specifies exactly how my datetimes are to be formatted. For example, '21 Jul 2026' vs '21-07-2026'.

If the data is represented as a dtype object (containing string values), then SDV can explicitly validate that the provided format matches the data.

However if for some reason the data is represented as a datetime64, then there is no format really associated with those values. So in this case, SDV does not explicitly validate whether the data matches any explicitly provided format string. But as a user, I don't know that this validation is skipped.

Expected behavior

The metadata.validate_data functions should raise a warning if:

• The metadata has a specified datetime_format string for a particular column AND
• The data is represented as a dtype object (or any other dtype where the format cannot be checked)

The warning should specifically indicate that the datetime format cannot be verified and will be skipped.

Warning: The datetime format for column 'checkin_date' (table 'guests') could not be verified because the data is represented as dtype 'datetime64[ns]'.
Please omit the datetime format string from the metadata or cast the data to strings with the right format.