Trustworthy communication and co-operation requires in general static TTP services describing status and relation of communicating principals as well as their corresponding keys and attributes. Additional TTP services are needed to provide trustworthy information about dynamic issues of communication and co-operation such as message identifier, time and location of processes, workflow relations and system behaviour. Certified time stamps are important notary's functions dealing with the services mentioned above. Within the European RESHEN project, the ONCONET regional health network for improving cancer care has been moving from system-related to certified time stamps using a local secure time stamp server or deploying remote time stamp services provided by an accredited CA. After implementing such services at the beginning of 2002, an evaluation of the operation modes will be performed during 2002 to derive recommendations for optimisation of these important services in national and international health networks.