bdate — Byzantine time reckoning

bdate is date(1) for someone living in Constantinople a thousand years ago.

$ bdate
Sabbaton, 27 Septembrios 7535 AM (Julian 2026-09-27), indiction 5, 6th hour of the day

This line has five parts. Each comes from a separate Byzantine convention, and bdate stacks them in this order:

# Layer What it answers Page
1 Julian calendar Which day is it? The Julian calendar
2 Anno Mundi era Which year is it? Anno Mundi
3 Indiction Where are we in the 15-year cycle? The indiction
4 Greek names What are the day and month called? Days and months
5 Seasonal hours What hour is it? Seasonal hours

You can read these pages in any order. Each page explains the historical convention, gives the formula bdate uses, and works through an example you can reproduce.

Open the live clock → It shows the current seasonal hour on a dial, for Constantinople or anywhere else, and gives you code to embed it in your own page.

How the layers fit together

civil instant (your clock, any time zone)
        │
        ├──► Julian Day Number ──► Julian calendar date ──► month name
        │                     │                       │
        │                     └──► weekday name        └──► Anno Mundi year ──► indiction
        │
        └──► sunrise / sunset at (lat, lon) ──► seasonal hour (+ night watch)

The date layers use whole days only. The hour layer is the only part that depends on where you are. It defaults to Constantinople (Hagia Sophia, 41.0086° N, 28.9802° E).

A worked example: the fall of the City

Constantinople fell on Tuesday, 29 May 1453 in the Julian calendar, which is how contemporary sources record it. bdate reads its input as a modern (proleptic Gregorian) date, so you enter the Gregorian equivalent, which is nine days later:

$ bdate -d 1453-06-07T06:00+02:00
Tritē, 29 Maios 6961 AM (Julian 1453-05-29), indiction 1, 2nd hour of the day

Each part checks out against the sources:

Further reading