Abstract: A method of providing a time stamping service for setting a client's system clock, wherein the client's system clock can be set to reliable standard time. Whether the client's system clock has been accurately set is one of the preconditions for the smooth provision of a public key infrastructure-based security service. A service provider providing the public key infrastructure-based security service safely receives reference time information from an objectively reliable third-party system, sets the client's system clock on the basis of the received reference time information and provides the security service on the basis of the set client's system clock. Therefore, the service provider can smoothly provide the security service while assuring the accuracy and reliability of the client's system clock. Further, the smooth provision of the security service can reduce the number of petitions from clients to the security service, thereby increasing the quality of the security service.