Abstract. Calendar service protocol stack consists of four protocols: WebDAV, CalDAV, iCalendar and jCal. The basic functions of calendar service include personal planning, schedule sharing, events reminder, invitation and data synchronization among platforms or devices. Calendar service has a lot of commercial and open-source implementations such as Google calendar, Apple calendar, DAViCal, SabreDAV and Radicale. Role-based authorization and data synchronization are the two key technologies discussed. Problems like data cleansing, batch operation interfaces and cross-domain issues are given concerns, too.