The Spanish era or era of Caesar was a dating system commonly used in the states of the Iberian peninsula until the 14th–15th centuries, when it was phased out in favour the Anno Domini system. The calendar began with year one in what is 38 BC, possibly the date of a new tax imposed by the Roman Republic on the subdued population of Iberia. Whatever the case, the date signifies the beginning of the Pax Romana in Iberia. To convert from an Anno Domini date to the corresponding year in the Spanish era, add 38 to the Anno Domini year. A date in the Spanish era was written thus: "Era CMXLI" for "anno domini 903".
The use of the Caesar Era probably began in Iberia in the 3rd century. The reason for its particular popularity is unknown. Official usage ceased in different parts of the Iberian peninsula at different times: in Aragon in 1349, Valencia 1358, Castile 1383 and Portugal 1422. While the year officially began on 1 January under the Spanish era, that was changed to 25 December when the Anno Domini system was adopted. (The church, however, used 11 January.)
- Roth, Norman. "Calendar." Gerli, E. Michael. (Ed.). Medieval Iberia An Encyclopedia. Routledge, 2003. ISBN 978-0-415-93918-8