gistfile1.php

add_filter( 'the_content', 'prefix_change_hello_to_morning' );
function prefix_change_hello_to_morning( $content ) {

	if ( is_admin() ) {
		return $content;
	}

	$post_id   = get_the_ID();
	$post_type = get_post_type( $post_id );

	if ( ! in_array( $post_type, array( 'post', 'page' ) ) ) {
		return $content;
	}

	$current_time = current_time( 'timestamp' );

	if ( $current_time > strtotime( '12:00am' ) && $current_time < strtotime( '12:00pm' ) ) {
		return str_replace( 'Hello', 'Good Morning', $content );
	}

	return $content;
}