This paper contextualises and interprets a text seldom addressed in Anglophone scholarship: De die natali (‘On the birthday’), written by Censorinus to celebrate his patron Caerellius’ birthday in 238 c.e. By exploring both gestation (natalis) and time measurement (dies), the work seeks to elucidate and isolate Caerellius’ birthday in time; it is the ultimate guide to his dies natalis. Despite a seemingly narrow focus, De die natali is best understood as part of a broad ‘spectrum’ of encyclopaedic texts, exemplifying the ‘totalising’ impetus of knowledge ordering in the Roman Empire, while simultaneously exposing the limits of such efforts. An interlocking set of tensions underlie the text, which resonate with other encyclopaedic projects — tensions between unity and plurality, centre and periphery, and the relationship between nature and culture. De die natali is both a product of, and commentary on, the conditions of human knowledge and the Empire's cultural diversity in the early third century.