An example of simple calendrical conversion can be seen in the calendar of the Maya. The Mayan calendar is a complex integration of cycles from 13 days, to twenty days, to the solar year to produce a great cycle of fifty-two years (Aveni 1989: 185–252). The calendar follows its own numerical logic but ethnographic evidence allows us to reconstruct the working of this calendar. We are therefore able to convert the Mayan scale with its unique relationship with the solar year and convert it into the modern luni-solar calendar. The success of conversion is because Mayans were considerate enough to use a numerical calendar which is easily interpreted by modern investigators.
Considerate is not a word frequently applied to the Romans, and their calendar poses more serious problems for archaeologists. In the later, imperial, period the basis of the calendar were the reigns of the emperors. In the republican period it was the names of the consuls that gave the name to the year. Therefore to convert Roman dates, or any dates which rely upon the reigns of rulers, we must have access to an accurate historical record to allow us to trace the dates.
Unfortunately the earliest history, that of Herodotus, dates only as far back as the late fifth century. Beyond this date there may be records of rulers, but they become more susceptible to historical drift as date-keeping errors accumulate. For this reason while we can trace the earliest dynasty of Egypt back to the middle of the fourth millennium (Renfrew & Bahn 1996: 123), the error involved in doing so is considerable, being around two centuries. For prehistoric time, or artefacts which cannot be dated historically we need a system of time-keeping independent of human action.
For other methods of dating see the contents page of the series Dating for Archaeologists.