<P> The term calendar itself is taken from the calends, the term for the first day of the month in the Roman calendar, related to the verb calare "to call out", referring to the calling or the announcement that the new moon was just seen . Latin calendarium meant "account book, register", as accounts were settled and debts were collected on the calends of each month . </P> <P> The Latin term was adopted in Old French as calendier and from there in Middle English as calender by the 13th century . The spelling calendar is from Early Modern English . </P> <P> A number of prehistoric structures have been proposed as having had the purpose of timekeeping (typically keeping track of the course of the solar year). This includes many megalithic structures, and reconstructed arrangements going back far into the Neolithic period . </P> <P> A ceramic artefact from Bulgaria, known as the Slatino furnace model, has been pronounced by local archeologists and media to be oldest known calendar representation, a claim not endorsed in mainstream views . </P>

When did humans start keeping track of years