Retrieves post published or modified time as a Unix timestamp.

Note that this function returns a true Unix timestamp, not summed with timezone offset
like older WP functions.

function get_post_timestamp( $post = null, $field = 'date' ) {
	$datetime = get_post_datetime( $post, $field );

	if ( false === $datetime ) {
		return false;
	}

	return $datetime->getTimestamp();
}