Large software compilations based on free, open source software (FOSS) packages are the basis for many software systems. When they are deployed in production, specific versions of the packages in the compilation are selected for installation. Over time, those versions become outdated with respect to the upstream software from which they are produced, and from the components available in the compilations as well. The fact that deployed components are outdated is not a problem in itself, but there is a price to pay for not being “as much updated as reasonable”. This includes bug fixes and new features that could, at least potentially, be interesting for the deployed system. Therefore, a balance has to be maintained between “being up-to-date” and “keeping the good old working versions”. This paper proposes a theoretical model (the “technical lag”) for measuring how outdated a system is, with the aim of assisting in the decisions about upgrading in production. The paper explores several ways in which technical lag can be implemented, depending on requirements. As an illustration, it presents as well some specific cases in which the evolution of technical lag is computed.