During the years immediately following WWII, several groups of American and British physicists devoted themselves to the task of developing atomic clocks. Building upon frequency standardization techniques developed for radar during the war, groups working within institutions such as the US National Bureau of Standards, MIT, and the UK National Physics Laboratory aspired to create machines that would transform the way time was measured and defined. Close attention to this early work on atomic timekeeping in the late 1940s and early 1950s reveals a multiplicity of concepts, tools, and practices at play. Of particular interest is the variety of ways in which ‘time’ — the quantity to be measured by the new atomic clocks — was defined and discussed by the physicists involved. This paper traces the early years of atomic timekeeping, paying special attention to how ‘time’ was understood by the actors, with several objectives in mind. First, it seeks to show that the priority disputes surrounding the invention of the atomic clock were rooted in conceptual disagreements over the nature of time and timekeeping. Second, it argues that the way the actors understood time provides insight into their self-image as physicists, and their understandings of their discipline. Finally, the paper considers the vast amount of media attention paid to atomic clocks during the 1950s and 60s, and they ways this coverage affected and reflected how physicists and the public understood time and physics.