Takayuki Miyoshi on "[Plugin: Contact Form 7] validation issue in custom field and custom shortcode"

add_filter('wpcf7_custom_date',  'custom_date_filter', 10, 2);
add_filter('wpcf7_custom_date*', 'custom_date_filter', 10, 2);

These filter names should be wpcf7_validate_custom_date and wpcf7_validate_custom_date* respectively.