The language of partial order time expresses the issues central to many problems in asynchronous distributed systems. A secure partial order time service would provide a general method to develop secure protocols for these problems. In this paper, we sketch out these issues and develop one such protocol: signed vector timestamps. The majority of this paper is drawn verbatim from the first author's October 1991 thesis proposal, the first research into security issues for non-scalar time services and the original presentation of the SVT protocol. This research was sponsored by the Avionics Laboratory, Wright Research and Development Center, Aeronautical Systems Division (AFSC), U. S. Air Force, Wright-Patterson AFB, OH 45433-6543 under Contract F33615-90-C-1465, Order No. 7597. S. Smith also received support from an ONR Graduate Fellowship and J.D. Tygar from NSF Presidential Young Investigator Grant CCR-8858087. The views and conclusions contained in this document are those of the autho..