[1.] Introduction; [2.] Anno Domini dating; [3.] Bissextile; [4.] Anno Domini dating in AU; [5.] The origin of March-Anno Domini dating; [6.] Ferials and epacts; [7.] Au’s list of papal succession; [8.] Bissextile data; [9.] Anno Mundi; [10.] Pre-AD dating of records in Ireland; [11.] Conclusion: AU includes two different dating systems. Discusses the historical background for the adoption of 1 January or 25 March as the date for the beginning of the year.