"Since Your Last Visit" Dashboard Widget

add_action( 'admin_init', function () {
    if ( wp_doing_ajax() ) {
        return;
    }
    $user_id = get_current_user_id();
    $last    = (int) get_user_meta( $user_id, 'wpide_last_admin_visit', true );

    // Move the "previous visit" marker forward after an hour away.
    if ( $last < time() - HOUR_IN_SECONDS ) {
        update_user_meta( $user_id, 'wpide_prev_admin_visit', $last ? $last : time() );
        update_user_meta( $user_id, 'wpide_last_admin_visit', time() );
    }
} );

add_action( 'wp_dashboard_setup', function () {
    wp_add_dashboard_widget( 'wpide_since_last_visit', 'Since your last visit', function () {
        $since = (int) get_user_meta( get_current_user_id(), 'wpide_prev_admin_visit', true );
        if ( ! $since ) {
            echo '<p>Welcome! Activity will appear here from your next visit.</p>';
            return;
        }
        $after = gmdate( 'Y-m-d H:i:s', $since );

        $posts = get_posts( array(
            'post_type'      => array( 'post', 'page' ),
            'posts_per_page' => 8,
            'no_found_rows'  => true,
            'author__not_in' => array( get_current_user_id() ),
            'date_query'     => array( array( 'column' => 'post_date_gmt', 'after' => $after ) ),
        ) );
        $comments = get_comments( array( 'status' => 'approve', 'count' => true, 'date_query' => array( array( 'column' => 'comment_date_gmt', 'after' => $after ) ) ) );
        $pending  = get_comments( array( 'status' => 'hold', 'count' => true, 'date_query' => array( array( 'column' => 'comment_date_gmt', 'after' => $after ) ) ) );
        $users    = current_user_can( 'list_users' ) ? count( get_users( array( 'fields' => 'ID', 'date_query' => array( array( 'column' => 'user_registered', 'after' => $after ) ) ) ) ) : 0;

        printf( '<p>Since %s ago:</p><ul>', esc_html( human_time_diff( $since ) ) );
        printf( '<li><strong>%d</strong> new comments%s</li>', (int) $comments, $pending ? ' (<a href="' . esc_url( admin_url( 'edit-comments.php?comment_status=moderated' ) ) . '">' . (int) $pending . ' awaiting moderation</a>)' : '' );
        if ( current_user_can( 'list_users' ) ) {
            printf( '<li><strong>%d</strong> new users</li>', (int) $users );
        }
        printf( '<li><strong>%d</strong> new posts and pages by others</li></ul>', count( $posts ) );
        if ( $posts ) {
            echo '<ul style="margin-left:1em;list-style:disc">';
            foreach ( $posts as $post ) {
                printf( '<li><a href="%s">%s</a> <small>by %s</small></li>', esc_url( get_permalink( $post ) ), esc_html( get_the_title( $post ) ), esc_html( get_the_author_meta( 'display_name', $post->post_author ) ) );
            }
            echo '</ul>';
        }
    } );
} );