Drawing on ethnographic fieldwork in data-intensive laboratories, this paper discusses the building of so-called biological clocks in personalized medicine. These consist of ageing biomarkers developed based on various omic data, such as epigenomics or glycomics, to monitor and compare individuals ‘true’ biological age against their chronological age. We start by analyzing what it takes to build such biological clocks, specifically pointing to the work of data analysts mobilizing different forms of data to assess individuals’ bodily ageing. We then explore the sort of futures, subjectivities and imaginaries, which are crafted in such testing practices. Using the concept of decay, we argue that biological clocks represent efforts to monitor individuals’ decay, and in particular it looks to identify when decay is ‘out of tempo’ to then offer personalized solution to slow down decay. In fact, many of these biological clocks are packaged as direct-to-consumer products that come with personalized lifestyle programs to help individuals slow down ageing. Beyond learning about the practical implications of building such biological clocks, this case is particularly revealing of the political vision of personalized medicine, and contemporary genomics more broadly. As biological clocks are made into marketed products measuring individuals’ health and wellness, personalized medicine becomes an individualized and biomedicalized endeavor looking to offer tailored solution to slow down or stop decay.