The early rulers of Western Han and Rome were faced with very different challenges in achieving the consolidation of one-man rule over vast territories and heterogeneous peoples. In Rome, the centre’s authority over peripheral territory was largely accepted, but the rule of one man was much derided. In the early Western Han, the opposite was true: while one-man rule had long been the norm, imperial authority over territory was tenuous, with portions of the empire under the rule of kings or regional lords (zhuhou), who ruled their lands and peoples autonomously, while acknowledging the emperor as the ritual centre of the empire. While the strategies undertaken by the early rulers in their respective territories to establish themselves as sole rulers, through the use of military and political means, differed greatly, there were also a number of similarities in the processes through which they consolidated their power, particularly in the ideological realm. One such similarity was that in both the Han and Roman empires, during these periods of consolidation, rulers initiated and completed reforms to the calendrical systems issued by the court. Despite the enormous differences between the two modes of calendrical computation, and the lack of direct contact between the two civilizations in this early period, there are a number of points where the processes through which they completed their calendar reforms look rather similar. Due to differences in the historiographical tradition and political circumstances, the scholars of Rome and the Han wrote about these reforms in very different ways, emphasizing or occluding information about the impact of these new calendars. By interrogating the Han sources with Roman questions of political importance, and the Roman sources with Han concerns over cosmic alignment and historical legitimacy, this chapter seeks to demonstrate some of the similarities between the calendrical reforms that a comparative approach reveals.