Chronograms are a cryptographic written practice in which the numerical values of some letters of a text encode a date relevant to that text. They constitute a form of ludic numeracy ‒ a specific kind of ludic language or wordplay in which writers not only highlight their skill with words and numbers but also conceal information, forcing readers to expend effort to extract their hidden meanings. Four distinct chronogram traditions are outlined: South and Southeast Asian word-symbols, South Asian alphasyllabic numerals; Greek, Hebrew, and Arabic alphanumeric chronography; and the early modern Western European tradition using Roman numerals. This chapter analyzes a corpus of over 10,000 Roman numeral chronograms from the fourteenth through the twentieth centuries, drawn from those previously compiled by the antiquarian James Hilton. Roman numeral chronograms use the letters IVXLCDM, specially marked within texts, to encode dates.While these chronograms began prior to the Hindu-Arabic (Western) numerals’ ubiquitous use, they were most popular in the late seventeenth and early eighteenth centuries, by which time the Roman numerals were already archaic. It was thus just at the point when Roman numerals were falling out of everyday use that they were ripe for symbolic repurposing in chronograms.