ANNO (CONSOLARE). — The practice of designating the years by the names of the ordinary consuls (not of the “sufferi”), whence their designation as “eponymous,” dates back to 509 B.C. and continued in the West until 534 and in the East until 541. Only from the year 153 B.C. did the beginning of the consular year begin to coincide with 1 January; nevertheless, the consular year did not always have its ordinary duration: in certain years it was sometimes only six months long or even shorter.
From 535 to 547 A.D. in the West, the formula was used: p(wt) (consulatum) Paulini iunioris I (up to XIII); in the East, from 542 to 565, the formula was used: p. c. Basilii iunioris I (up to XXIV). In 566, when Justin II reserved the title of consul to the emperor, the indication of the p. c. was subsequently added after the regnal years, less one unit: e.g. Data die X Kal. Augustorum, imperante d. n. Mauricio Tiborio piissimo Aug. an. XIV, p. c. eiusdem d. n. an. XIII, indictione XIV = 596 (S. Gregorii, Reg., VI, 50: Jaffé-Wattenbach, 1434).
Even in Christian monuments and ecclesiastical documents, the consular or post-consular date, until the eighth century, is the most common, as can be seen, for example, from the dated inscriptions ranging from the third to the seventh century (cf. list in E. Diehl, Inscriptiones lat. christ. veteres, III, Berlin 1931, pp. 229–63). The Byzantine formula mentioned above for dating, later adopted in the Liber Diurnus (ed. Th. E. von Sickel, Vienna 1889; cf. e.g. form. VII), helped to bring post-consular years into a certain number of papal documents up to the beginning of the tenth century, attributing them to the Carolingian emperors, since, having lost their true meaning, they were equated with regnal years.
The date of Paschal I’s privilege for the Church of Ravenna, of 11 July 819, is formulated as follows: unquato Hindovico a Deo coronato magno pacifico imperatore anno sexto, et [post] consulatum eius anno sexto (G. Battell, Exempla Scriptuorum, fasc. III: Acta Pontificum, Vatican City 1933, p. 5 and pl. 1).
The last testimony to this anomaly is the letter of Pope Sergius III, of 22 February 904, which mentions the IVth regnal year and the IVth post-consulate of Louis III (Jaffé-Wattenbach, 1533). The date with the name of the Byzantine emperor and his p. c. appears for the last time in Hadrian I’s privilege for Farfa of 772 (Jaffé-Wattenbach, 2395).