The essay examines Maya numeration and its specialized day-count system, distinguishing the general vigesimal scheme from the modified cycle used for counting days, in which the second place counts to eighteen to produce a 360-day unit. It argues that this day numeration functions as a calendar and critiques competing nomenclatures for units such as tun, katun, and ahau. Drawing on early Maya texts, especially the Books of Chilam Balam, it explains how katuns were named or numbered according to the day Ahau with which they ended, and advocates relying on contemporary evidence to settle terminological disputes.