Adding to what
@EnvoyToTheMolePeople and others have said…
By their very nature, dating systems using a ‘numerical’ date need a starting point as a reference for counting and are relative to that reference. So, for our current Western system we use the birth of Christ as the start point. You asked why that start point was designated as “zero” but in fact it isn’t. There is no “year zero” and the year following 1 BC is actually designated AD 1
The starting reference point for Anno Domini (the year of our Lord) uses the traditionally reckoned year of his birth which the Gospels only give us by reference to other events. There are two main clues: one in reference to the reign of King Herod; the other in reference to the beginning of Christ's preaching in the 15th year of the reign of Tiberius Caesar and subtraction of his stated age of about 30 years. Retrospectively, modern scholars believe the indicated year of birth would either be between 6 BC and 4 BC or around 2 BC depending on which biblical/historical evidence one uses (and for sure not on 25th December).
Contemporary events in the Roman world would have used the Julian Calendar established by Julius Caesar in 46 BC. Before that, their calendars used the foundation of Rome in 753 BC and the rule of King Romulus as the start point.
Any precise historical dates reckoned from our modern calendar also need to take account of the fact that we switched from the Julian calendar to the Gregorian calendar in 1582 following recalculation of the average solar year. The date was advanced by ten days such that, in 1582, Thursday 4th October was followed by Friday 15th October.
Calendars in other cultures and civilisations take different starting points derived from historical events, celestial events, birth or reigns of rulers/leaders and such. So, for example, our current year of 2025 would be:
7533/4 on the Byzantine calendar (starting from the creation era of Constantinople)
6775 on the Assyrian calendar (starting from the calming of the Great Flood)
5785/6 on the Hebrew calendar (starting from Old Testament creation of the world)
4722/3 or 4515 on the Chinese calendar (based on solar, lunar and other cycles)
1446/7 on the Islamic calendar (starting from Muhammad’s migration to Medina)
Truly ancient artefacts are not usually dated as such and we rely on context or stratigraphy, Marine Isotope Stages, Radiometrics, Thermoluminescence and other techniques.
The Chinese ‘Diamond Sutra’ is indeed the oldest book carrying a physical date, equivalent to 868 BC. However, the Egyptian ‘Prisse Papyrus’ dates to c.1800 BC based on its reference to the 27th year of Khufu’s reign. There are older texts, at least as far back as 3,500 BC, but they aren’t directly dated.
The oldest coin known to have a physical date is probably a Samian Silver Tetradrachm struck in Zankle (modern-day Messina) in Sicily. It carries the letter ‘A’ for ‘year one’ of their calendar, equivalent to 494 BC.
Just for interest, the white flecks in this slice from a carbonaceous chondrite meteorite in my collection are the oldest things I have that can be seen with the naked eye.
View attachment 2187106
They’re called calcium–aluminium-rich inclusions (CAIs) and are the first materials known to have existed in a solidified state as our solar system was being formed. For the four CAIs that have been dated using the Pb-Pb chronometer, the mean age was determined as 4567.30 ± 0.16 Million years. That’s consistent with the commonly accepted birth of our Solar System around 4.6 billion years ago.
