This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
The Indiction was used in the middle ages to specify the position of a
year in a 15 year taxation cycle. It was introduced by emperor
Constantine the Great on 1 September 312 and ceased to be used in
The Indiction may be calculated thus:
Indiction = (year + 2) mod 15 + 1
The Indiction has no astronomical significance.
The Indiction did not always follow the calendar year. Three different
Indictions may be identified:
1) The Pontifical or Roman Indiction, which started on New Year's Day
(being either 25 December, 1 January, or 25 March).
2) The Greek or Constantinopolitan Indiction, which started on 1 September.
3) The Imperial Indiction or Indiction of Constantine, which started
on 24 September.