The European Calendar was introduced in 2006. Everyone refers to it as EF but whether this is a phonetic reflection of their opinion, or whether it just seemed appropriate that the EF combination should follow on from BC and AD, no one seems quite sure. In either event, 2006 became EF1.
"A Futuristic Fantasy,"
Word Ways: Vol. 40
, Article 23.
Available at: https://digitalcommons.butler.edu/wordways/vol40/iss1/23