wp-includes/post.php:7391Checks for changed dates for published post objects and save the old date.
$post_idint$postWP_Post$post_beforeWP_Postfunction wp_check_for_changed_dates( $post_id, $post, $post_before ) { $previous_date = gmdate( 'Y-m-d', strtotime( $post_before->post_date ) ); $new_date = gmdate( 'Y-m-d', strtotime( $post->post_date ) ); // Don't bother if it hasn't changed. if ( $new_date === $previous_date ) { return; } // We're only concerned with published, non-hierarchical objects. if ( ! ( 'publish' === $post->post_status || ( 'attachment' === $post->post_type && 'inherit' === $post->post_status ) ) || is_post_type_hierarchical( $post->post_type ) ) { return; } $old_dates = (array) get_post_meta( $post_id, '_wp_old_date' ); // If we haven't added this old date before, add it now. if ( ! empty( $previous_date ) && ! in_array( $previous_date, $old_dates, true ) ) { add_post_meta( $post_id, '_wp_old_date', $previous_date ); } // If the new slug was used previously, delete it from the list. if ( in_array( $new_date, $old_dates, true ) ) { delete_post_meta( $post_id, '_wp_old_date', $new_date ); }}Introduced in 4.9.3. Unchanged from 6.7.7 through 7.1.0.
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src/wp-includes/post.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.