This paper critically examines the Google Calendar. For this purpose, the offered functions of the core product are studied on privacy aspects. On one hand, it is identified, to which extent the product could infringe the users" privacy, on the other accuring risks are discussed. Furthermore, the functions in terms of their use for both, the service provider Google and for the user, are considered. A detailed analysis demonstrates the critical aspects, in which we have to decide between privacy and functionality. The identified solutions to minimize discussed risks of IT security mechanisms, are presented, discussed and analyzed in terms of their feasibility. Afterwards the individual solutions are summarized in a security concept and other requirements are explained. Finally, a Firefox-Addon which implements the described solution concept should be created, to resolve the existing weaknesses to the best of its ability. Ultimately, the functionality of the addon with technical implementation is illustrated in detail.