A.R., meaning "After Reconstitution," is the basis for the calendar used on Arbre. The year of the Reconstitution is defined as the year 0 A.R. The novel Anathem takes place during the years 3689 A.R. to 3690 A.R.
Unlike the vast majority of calendars in use on Earth, the calendar on Arbre includes a year zero. Years before the Reconstitution are "negative," for example The Terrible Events occurred in the year -5 ("negative five") A.R.