BC Dating?

Most coins are not dated. Dating is a lot lot later. The current dating system isn't going to come about for a while either.

However, some coins ARE dated. These have the date compared to another date - for example, a roman coin might be dated 'Year 7'. this doesn't mean 7AD, this means the 7th year of the reign of the guy who issued it. This means if he started his rule in march then the date refers to any date between march one year and march the next year.

I must stress though, very few coins are dated - even that system doesn't match a date.

Much of this is borrowed from Wiki but they say it better than I could

It refers only to the Romans, I believe all civilisations had their own systems usually starting at a 'year 1' at an important event or founding.

It is not known how years were identified during the Roman monarchy. During the Roman Republic, years were named after the consuls who were elected annually. Thus the name of the year identified a consular term of office, not a calendar year. For example, 205 BC in our system was to them 'The year of the consulship of Publius Cornelius Scipio Africanus and Publius Licinius Crassus'. Lists of consuls were maintained in the fasti. The fasti was a chronological or calendar-based list of official and religiously sanctioned events.


Fasti

The first day of the consular term changed several times during Roman history. It became 1 January in 153 BC. Before then it was 15 March. Earlier changes are a little less certain. There is good reason to believe it was 1 May for most of the 3rd century BC, until 222 BC. Livy mentions earlier consular years starting on 1 Sextilis (August), 15 May, 15 December, 1 October and 1 Quintilis (July).

In the later Republic, historians and scholars began to count years from the founding of the city of Rome. Different scholars used different dates for this event. The date most widely used today is that calculated by Varro, 753 BC, but other systems varied by up to several decades. Dates given by this method are numbered ab urbe condita (meaning from the founding of the city, and abbreviated AUC), and correspond to consular years.

I believe to get dates meaningful to us (29 BC) we have to work back from the founding of the Gregorian calendar (the one we still use today, roughly) in 1582 and use the previous calendars (eg Julian calendar) to help get the scale correct.

On the actual coins as Ben and echizento have already stated usually a reignal year or consul year was sometimes included, at certain times the Emperors were coming and going so quickly simply a name was enough to give a tight date.

An interesting question that I'd never really thought about jpsned

collecting errors is so much easier

That's why you do it.

Wow! Fascinating. I guess this brings up the larger question, before the year "0" in our AD calendar, how did the ancients officially refer to the passage of time in their dating systems?

As has been alluded to in previous posts, the AD calendar we use today was not invented until long after the event it marks as Year 1 took place. Dionysius Exiguus, a monk living around AD 525, invented the calendar to replace the calendar used by the Eastern Church up until that time, the Era of Martyrs calendar which set Year 1 as the accession of emperor Diocletian, the last great persecutor of the church, in AD 284. Apparently, Exiguus believed that perpetuating the memory of Diocletian was something to be avoided.

AD dates did not actually appear on coins until much later, the first being a coin from Denmark dated 1234.

Regnal-date calendars are the oldest calendar style, and are still used in some countries: Japanese coins, for example, are dated according to the regnal year of the Emperor.