The years 783-844 in Harleian MS 3859.
including Irish and North British. The first entry corresponds to the year 453. The word 'corresponds' is used because these terse annals do not actually quote dates. The sporadic entries are made on a framework of years - each year marked only by an abbreviation of the word 'annus'. Every tenth year is additionally identified with a Roman numeral (though some decades have nine years, some eleven). Although the first entry corresponds to 453, the framework begins at the equivalent of 445, and, although the last entry corresponds to 954, the framework extends to the equivalent of 977. The significance of the start date can only be guessed at, but it seems likely that the span of the framework is based on the Easter cycle of 532 years. The Harleian MS 3859 version is the A-text of the 'Annales Cambriae'. The B-text and C-text exist in late-13th century manuscripts.The C-text (British Library MS Cotton Domitian A i) was produced at St.Davids. It begins with a world-history section, ultimately derived from the work of Isidore of Seville (d.636), which continues to the reign of the emperor Heraclius (610-41). Merged into the Isidoran section are events which appear in the A-text, and also material from Geoffrey of Monmouth (whose influence continues up to 734). After Heraclius, the framework of years, marked by the word 'annus', appears (although there is no separate indication of decades), and continues to the equivalent of 1289 (the last entry actually appears against the previous 'annus' - corresponding to 1288).
Geoffrey of Monmouth's imaginative 'Historia Regum Britanniae' (History of the Kings of Britain) was written in the 1130s. In a 'Sir John Rhys Memorial Lecture' from 1973 (published in the 'Proceedings of the British Academy'), Kathleen Hughes says: "The Cottonian annals (i.e. the C-text) down to 689 are valueless as history, but they show the enormous influence of Geoffrey not only on romance but on historical writing."
The B-text is written on the flyleaves of an abbreviated edition of the 'Domesday Book' (Public Record Office MS E.164/1). It was probably written at the Cistercian abbey of Neath. It too begins with an Isidoran section, but the framework of years begins with Julius Caesar's invasion of Britain - though the Isidoran contribution continues up to the reign of Leo I (457-73). Up to 682, the influence of Geoffrey of Monmouth is apparent. Like the C-text, the B-text also does not indicate decades, however, from 1097 until the final entry, in 1286, Anno Domini dates are attached to the annals.
The A-text may be the most ancient surviving version of the 'Annales Cambriae', but it does not represent the original compilation (though its exemplar cannot have postdated that original by much). It appears that a common St.Davids text was the source for both the B-text and C-text (up until 1202, after which they diverge), however, that St.Davids text was independently derived from an ancestor of the A-text.
Another 13th century manuscript (Exeter Cathedral MS 3514) contains two other texts. The second of these swiftly covers the years 1132BC - AD1285, and has, asserts Kathleen Hughes (whose 1973, British Academy, lecture was entitled 'The Welsh Latin Chronicles: Annales Cambriae and Related Texts') "no independent historical value". Much more important is the manuscript's other text, known as the 'Chronica de Wallia', which, since it does not start until 1190, is not relevant to this website.
Perhaps unsurprisingly, the frameworks of the three texts do not stay in register with each other, and none of them stays in register with dating references derived from other (reliable) sources. Apart from simple counting errors (as exemplified by the A-text's nine and eleven year decades), it is easy to imagine, when there is nothing to distinguish one year from another, the ease with which events could be misplaced on the framework. As a result, deriving accurate dates from the 'Annales Cambriae' is something of an inexact science.
Another, related, Welsh source, is the 'BRUT Y TYWYSOGION' (Chronicle of the Princes). This is basically a translation into Welsh of a, no longer extant, Latin text - based on the 'Annales Cambriae', but with additional material - compiled in the late-13th century. The 'Brut y Tywysogion' exists in a number of texts produced at different sites - the first entry corresponding to the year 682. In the pre-Conquest period, there are occasional remarks which do not appear in any of the surviving texts of the 'Annales Cambriae'.