Wireless sensor networks are gaining popularity. They are used in consumer and industrial applications, including home automation, healthcare applications, industrial process monitoring and control, environment and habitat monitoring, and traffic control. The sensor nodes in a network can capture sensitive and valuable information. This information is collected in a central server for further analysis. The transmission of these data needs to be secured and therefore, the research community is focusing on the security protocols to achieve this, and on the underlying threat model. In this paper we focus on the secure logging of the information gathering from the different nodes. The goal is to prove the chronological order, in a reliable way, of the observations and events in nodes. It can be useful to prove that a certain event at node x happened before another event at node y. For instance, consider the setting of a wireless body area sensor network (WBAN) in a hospital. A secure logging of the outputs of sensors from different patients enable the study of the flow and effects from the hospital bacteria, and leave no possibility to re-order events afterwards. Our proposal is based on the notion of linked time-stamps, in which a mes- sage digest chain is formed through events to be logged. The linking takes place at so-called gateway nodes, acting as communication hubs, and the central server node. Consequently, the gateway node becomes responsible for ordering the in- formation from the sensors of its network, while the central server orders the information of the different gateways.