computus
/kəmˈpjuːtəs/(also compotus) Now historical
noun
1A calculation, a reckoning; an account; a record of accounts. In later use frequently attributive.
2A set of tables used in medieval times for calculating astronomical phenomena and the movable dates of the calendar; a calendar.
Origin
Mid 17th century. From post-classical Latin computus (also compotus) calculation, computation, calculation of Easter, calendar, chronology, account (frequently from 1086 in British sources) from classical Latin computāre.