How to sort by a field that is a date on the pre_get_posts hook?

To sort posts by a custom date field using the pre_get_posts hook, you need to adjust the query parameters with the $query->set() method.

Here’s an example:

function sort_posts_by_date_field( $query ) {
    // Ensure this is the main query and not in the admin area
    if ( ! is_admin() && $query->is_main_query() ) {
        // Make sure this is the correct post type or archive
        if ( $query->is_post_type_archive( 'your_post_type' ) || $query->is_tax( 'your_taxonomy' ) ) {
            // Specify your custom date field meta key
            $meta_key = 'your_date_field';

            // Adjust the query parameters
            $query->set( 'meta_key', $meta_key );
            $query->set( 'orderby', 'meta_value' );
            $query->set( 'meta_type', 'DATE' ); // Set the field type to DATE
            $query->set( 'order', 'ASC' ); // Or DESC for descending order
        }
    }
}
add_action( 'pre_get_posts', 'sort_posts_by_date_field' );

Explanation:

1. Query checks:
   • is_admin() ensures the code doesn’t run in the admin area.
   • is_main_query() checks if this is the main query (to avoid affecting secondary queries).
2. Archive type checks:
   • is_post_type_archive( 'your_post_type' ) ensures it’s a specific post type archive.
   • is_tax( 'your_taxonomy' ) ensures it’s a taxonomy archive.
3. Sorting:
   • meta_key — the meta key of your custom field.
   • orderby — specifies sorting by the meta field value.
   • meta_type — specifies the type of the meta field as DATE.
   • order — specifies the sort order (ASC for ascending or DESC for descending).

Example with a real field:

If you have a custom field with the key event_date, replace 'your_date_field' with 'event_date'.

Notes:

• Make sure the date field values are stored in the YYYY-MM-DD format for proper sorting.
• If the date format differs (e.g., dd-mm-yyyy), you may need to convert the values before using them in the query.