Secure relative timestamping and secure append-only logs are two historically mostly independent lines of research, which we show to be sides of the same coin -- the authentication of prefix relations. From this more general viewpoint, we derive several complexity criteria not yet considered in previous literature. We define transitive prefix authentication graphs, a graph class that captures all hash-based timestamping and log designs we know of. We survey existing schemes by expressing them as transitive prefix authentication graphs, which yields more compact definitions and more complete evaluations than in the existing literature.