By the end of the eleventh century the most ambitious attempt to overthrow the Dionysian chronology of the post-incarnation era had been made. An Irish monk known as Marianus Scottus (1028–82) produced a voluminous work, the Chronicon, expressing his views on Christian chronology. Analysing the Gospels and the calculations of his predecessors, Marianus corrected Dionysian chronology by twenty-two years. Unlike other corrections proposed since the tenth century, Marianus's project went beyond the limits of pure computistical theory. It included a substantial historical excursus in which Marianus reconstructed the correct chronology year by year and rediscovered these ‘lost’ twenty-two years.

The influence of this major historical project on Anglo-Norman historiography cannot be overestimated. Marianus's text, directly or indirectly, inspired other authors to create their own works and served as an important source for them. However, the main claim of his work was not generally accepted. Although Marianus's emendation of the Dionysian Christian era had the widest dissemination of all other corrections, Marianus failed to achieve his main goal; the work's impact was not strong enough to replace the existing chronological system. This was largely the result of the complexity of the subject, but also of the confused manner in which Marianus expressed his calculations, which made his conclusions difficult to decipher. In this context, the work written in 1086 by Robert, bishop of Hereford (1079–95), was of particular importance. The Excerptio Rodberti Herefordensis episcopi de Chronica Mariniani, as it was entitled by William of Malmesbury, served as an indispensable instrument that allowed intellectuals to familiarize themselves with the ideas of Marianus.

Robert's 24-chapter work, written in 1086, is largely drawn from the computistical preface to the Chronicon of Marianus Scottus as well as from its first two books. Robert's own textual additions and changes are limited and do not alter the sense of Marianus's conclusions. However, the clearer composition and brief form of Robert's treatise made it much more understandable than the original source.