1. AD Dates started becoming common on coins in the 1400s, but the first AD-dated coin is from 1234.
2. The earliest dates in absolute terms come from Greek coinage. Some Seleucid Greek coins (modern day Middle East, Turkey to Afghanistan) bear a date (in Greek numerals) relating to the founding of their empire in 312 BC.
Some other old calendar systems commonly used on coins:
- Most Arab coins (at least the silver and gold ones) bear a date beginning with the standardized coinage reform in 76 AH, or 695 AD. These dates are written out longhand until the early modern age, however
- The Byzantine, Sassanian, and Roman empires, in addition to many smaller kingdoms, commonly used regnal years on coins. (Caveat: Romans only had their regnal years spelled out on certain provincials, although early imperial coins used TR P, or granting of Tribunician power, which had to be renewed every December 9.)
- A small kingdom commonly called the Western Satraps in India used a standard calendar system in Brahmi numerals relating to the founding of their kingdom in 78 AD. I believe this is the earliest calendar system to use numerals in a base-10 numbering system to denote a date.
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