Source

File: wp-includes/comment.php

function get_lastcommentmodified( $timezone = 'server' ) {
	global $wpdb;

	$timezone = strtolower( $timezone );
	$key      = "lastcommentmodified:$timezone";

	$comment_modified_date = wp_cache_get( $key, 'timeinfo' );
	if ( false !== $comment_modified_date ) {
		return $comment_modified_date;
	}

	switch ( $timezone ) {
		case 'gmt':
			$comment_modified_date = $wpdb->get_var( "SELECT comment_date_gmt FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1" );
			break;
		case 'blog':
			$comment_modified_date = $wpdb->get_var( "SELECT comment_date FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1" );
			break;
		case 'server':
			$add_seconds_server = gmdate( 'Z' );

			$comment_modified_date = $wpdb->get_var( $wpdb->prepare( "SELECT DATE_ADD(comment_date_gmt, INTERVAL %s SECOND) FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1", $add_seconds_server ) );
			break;
	}

	if ( $comment_modified_date ) {
		wp_cache_set( $key, $comment_modified_date, 'timeinfo' );

		return $comment_modified_date;
	}

	return false;
}