The Red Book of Hergest claims there were 128 years from the start of Vortigern’s reign to the battle of Badon. As Vortigern’s reign started in the year 425 and Badon was in 518, this is clearly incorrect. The question is how was the figure of 128 arrived at. It is likely to have been as a result of the following:
1. The Incarnation and Passion was generally taken to be separated by 28 years. However, the source of the RBH, took 35 years as the time gap on the basis of HB 4, which states “From the Passion of Christ 796 years have passed; from the Incarnation 831 years.”.
2. The start of Vortigern’s reign was 425 AD. However, in the earlier dating method it would have been 390 AP (anno passionis) using the 35 year interval.
3. By the time of the RBH, the figure 390 had been interpreted by the later dating method as 390 AD (anno domini).
4. The RBH statement “From the age of Vortigern to the Battle of Badon, which Arthur and his nobles fought with the Saxons, when Arthur and his nobles were victorious, 128 years.” used the fallacious calculation 518 – 390 = 128.