Usage of Direct Database Call Without Caching Error

I agree with @Tom J Nowell:

I’d suggest you remove it, or better yet just say “all rights reserved”.

But in the quest for answers, here’s a suggestion using an option field to cache the years:

/**
 * Get year range for posts.
 * 
 * @return str
 */
function wpse_226627_get_copyright() {
    if ( ! $years = get_option( 'copyright' ) ) {
        $args = [
            'posts_per_page' => 1,
            'post_type'      => get_post_types([ 'public' => true ]),
            'post_status'    => 'publish',
            'orderby'        => 'post_date',

            /**
             * Don't waste memory we don't need
             */
            'update_post_term_cache' => false,
            'update_post_meta_cache' => false,
            'cache_results'          => false,
        ];

        $newest = get_posts([ 'order' => 'DESC' ] + $args );
        $oldest = get_posts([ 'order' => 'ASC'  ] + $args );
        $years  = [
            'from' => $oldest ? mysql2date( 'Y', $oldest[0]->post_date_gmt ) : '',
            'to'   => $newest ? mysql2date( 'Y', $newest[0]->post_date_gmt ) : '',
        ];

        update_option( 'copyright_years', $years );
    }

    return $years;
}

/**
 * Bust the cache.
 */
function wpse_226627_flush_years( $post_id ) {
    if ( in_array( get_post_type( $post_id ), get_post_types([ 'public' => true ]) ) )
        update_option( 'copyright_years', '' );
}

add_action( 'before_delete_post', 'wpse_226627_flush_years' );
add_action( 'save_post',          'wpse_226627_flush_years' );

This way the cache is flushed & populated only when a post is created/updated/deleted, so a couple of extra queries (instead of one direct database call) is no real biggy here.

$years     = wpse_226627_get_years();
$copyright = $years['from'];
if ( $years['from'] != $years['to'] )
    $copyright .= ' &mdash; ' . $years['to'];

I chose to cache the years as a serialized array (as opposed to the compiled copyright string) so that you’ve always got the values to work with independently, should you ever wish to change their layout/do something differently with them.

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