The increase in road traffic accidents in twentieth-century Britain brought with it a rise in the number of patients admitted to hospital with blunt, non-penetrating head injuries. Patients who had suffered mild to moderate trauma typically complained of a variety of problems, including headaches, dizziness and giddiness. For the neurologists tasked with diagnosing and treating these patients, such symptoms proved difficult to assess and liable to obscure the clinical picture. This article focuses on why neurologists turned to time as a diagnostic-tool in helping to resolve these issues, specifically the measurement of post-traumatic amnesia (PTA). This article argues that PTA became so central to neurological diagnosis owing to a set of epistemic, professional and material factors in the decades prior to the Second World War. It concludes with a call for deeper appreciation of the range of issues that contribute to the shaping of medical theories of head trauma.