The indiction
History
The indiction (ἰνδικτιών) was originally a 15-year cycle of tax assessment, introduced under Diocletian and Constantine in the late 3rd and early 4th centuries. Each year was numbered 1 to 15 within the current cycle. The cycles themselves were not numbered.
Justinian made indiction dating compulsory on official documents in 537. For centuries it was the most common way Byzantines dated anything, often alone: “in the month of May, of the 1st indiction”. The indiction year began on 1 September. This is why the Byzantine year as a whole also began then.
Since the cycles carry no number, an indiction only identifies a year within a 15-year window. Historians therefore pair it with an era year (AM) or a regnal year to fix a date exactly.
How bdate computes it
The usual modern reconstruction ties the indiction directly to the AM year:
indiction = AM mod 15 (a remainder of 0 means indiction 15)
AM and the indiction both start on 1 September, so they change on the same day.
This formula matches well-known reference points:
| Event | AM | AM mod 15 | Indiction (sources) |
|---|---|---|---|
| Fall of Constantinople, 1453 | 6961 | 1 | 1 ✔ |
| 1 Sep 2026 – 31 Aug 2027 | 7535 | 5 | 5 |
Caveats
- Byzantine practice was not always consistent. Scribes sometimes kept the
old indiction for a while after 1 September, and some areas (in the West, the
“Bedan” or “Pontifical” indictions) used other start dates such as
24 September, 25 December or 1 January.
bdateuses only the Constantinopolitan 1 September indiction.