In the English constitutional tradition, subjecthood has been primarily derived from two circumstances: place of birth and time of birth. People not born in the right place and at the right time are not considered subjects. What political status they hold varies and depends largely on the political history of the territory in which they reside at the exact time of their birth. A genealogy of early modern British subjecthood reveals that law based on dates and temporal durations—what I will call collectively jus tempus—creates sovereign boundaries as powerful as territorial borders or bloodlines. This concept has myriad implications for how citizenship comes to be institutionalized in modern politics. In this article, I briefly outline one route through which jus tempus became a constitutive principle within the Anglo-American tradition of citizenship and how this concept works with other principles of membership to create subtle gradations of semi-citizenship beyond the binary of subject and alien. I illustrate two main points about jus tempus: first, how specific dates create sovereign boundaries among people and second, how durational time takes on an abstract value in politics that allows certain kinds of attributes, actions, and relationships to be translated into rights-bearing political statuses. I conclude with some remarks about how, once established, the principle of jus tempus is applied in a diverse array of political contexts.