 At the time Bede wrote the Historia Ecclesiastica , there were two common ways of referring to dates . One was to use <unk> , which were 15 @-@ year cycles , counting from 312 AD . There were three different varieties of indiction , each starting on a different day of the year . The other approach was to use regnal years — the reigning Roman emperor , for example , or the ruler of whichever kingdom was under discussion . This meant that in discussing conflicts between kingdoms , the date would have to be given in the regnal years of all the kings involved . Bede used both these approaches on occasion , but adopted a third method as his main approach to dating : the anno domini method invented by Dionysius Exiguus . Although Bede did not invent this method , his adoption of it , and his promulgation of it in De Temporum Ratione , his work on chronology , is the main reason why it is now so widely used . 
