Why there is no year zero, unless you ask an astronomer
The calendar most of the world uses was invented by a monk around 500 AD, extended backwards by Bede, and still skips straight from 1 BC to AD 1. Astronomers found that gap so awkward that in the 1700s they quietly adopted their own count, complete with a leap year zero.
Chronology is the discipline of putting events in their true order and pinning them to dates. It draws on timekeeping and on historiography, and it matters well beyond history, in astronomy, geology, palaeontology and archaeology. Its core problem is synchronisation: relating an event in one place to events elsewhere and to the present. Eusebius tackled exactly this in his Chronicon of 325 AD, which set the histories of many ancient kingdoms side by side in parallel columns.
Two ideas do the heavy lifting: a calendar, which organises days into years, and an era, which numbers those years from a starting point called an epoch. Dionysius Exiguus founded the Anno Domini era; Bede extended it to cover the years before Christ, with no zero. For around eight centuries it ran on the Julian calendar, then switched to the Gregorian after 1582. The Romans rarely counted from the founding of their city, traditionally 753 BC; they usually named a year by its two consuls. Later editors sprinkled that founding-year dating into Roman texts, making it look far more common than it was.
Where no written records exist, dates come from objects. Late nineteenth-century archaeologists built relative sequences from changing pottery styles, and in Egypt William Flinders Petrie developed seriation, ordering grave groups backwards from the earliest historical phases. Traded wares found at distant sites helped stitch regional sequences together.
After the mid-twentieth century, laboratory methods sharpened everything. Radiocarbon dating measures the share of carbon-14 remaining in once-living material, and tree-ring counting, matched against regional year-by-year sequences, is used to calibrate those radiocarbon results. The working rule is corroboration: dates from independent methods should check one another, and a conclusion resting on a single unsupported technique is generally treated as unreliable.
Source: Chronology
