My dissertation advances a novel interpretation of the work of twentieth-century political theorist Hannah Arendt, focusing on a neglected aspect of her thought: time. Arendt is well known for her analyses of how political disasters, including the loss of democratic freedom, follow from effaced spatial boundaries, i.e. the inappropriate merging of public and private spheres. My work reveals how Arendt's thought includes a framework for thinking about the significance of boundaries in time as well as space. I establish how and why those boundaries are politically created, describe what endangers their endurance, and reveal why freedom is undermined by such losses. Understanding Arendt's theory of time more clearly helps scholars to resolve apparent tensions in her thought, such as her emphases on novelty and stability in the political realm, as well as clarifying the relationship of Arendt's political theory to the philosophies of Kant, Nietzsche, Heidegger, and others. My research intervenes in the ongoing feminist reception of Arendt, including by demonstrating how her concept of time and theory of freedom both reckon with experiences of trauma. A theorist of hope as much as of political catastrophe, Arendt's work is particularly apropos in the precarious present we are collectively enduring.