Display Last Updated Date

<?php
add_filter( 'the_content', 'scseed_append_last_updated_date' );
if ( ! function_exists( 'scseed_append_last_updated_date' ) ) {
    function scseed_append_last_updated_date( $content ) {
        if ( ! is_singular( 'post' ) || ! in_the_loop() || ! is_main_query() ) {
            return $content;
        }
        $updated = get_the_modified_date( get_option( 'date_format' ) );
        $label   = '<p class="scseed-last-updated"><em>'
            . esc_html__( 'Last updated: ', 'scseed' )
            . esc_html( $updated )
            . '</em></p>';
        return $content . $label;
    }
}