Luckily, during Hellenistic times, when most formerly independent Greek cities had fallen under the authority of monarchs, the practice of placing dates on coins became commonplace in the eastern part of the Greek world. C., when the kingdom’s founder, Seleucus I, took possession of Babylon.
These are not dates that a modern collector will be familiar with, so you won't see a coin explicitly dated 100 B. The Seleucid era was so popular that it was used in other kingdoms, and even was used long after the Seleucids had left the scene.
This may explain why the ratio between year 1 and 2 is about 1:1 for 19 Alexandrian tetradrachms sold by Forum Ancient Coins.
The starting date for the coins of Galba minted in is still debated.
The earliest of these use a series of Phoenician symbols to represent the dates, whereas the later shekels and half shekels use the Greek letters shown on the table above, with year 1 starting in 126/5 B. The Ptolemaic system is relatively hard to decipher since most every dated silver coin uses the same design formula (portrait of the founder-king, Ptolemy I, and a standing eagle), and the coins often look quite similar from king to king, with there being only subtle differences in style and fabric that require intensive academic study to decipher. Some used the Greek alphabet with the letters Alpha through Omega representing the numbers 1-24 consecutively.
Some cities used Phoenician symbols, and the Nabataeans used their own numbering system.
And there are differences in the length of the first year.
The starting date of the famous Caesarean and Actian era is still subject of debate.And the moment of the first day of a certain year-count may differ as well. While modern western calendars start at the 1st of January, the Hebrew religious calendar, for example, starts in spring.Actually, the western names of the months for a part still refer to a time when the Roman calendar started the 1 about four millennia ago, like Adar for the last month of the year and Nisan for the first month in spring.For somebody born 31 January 1980, for example, the first year runs form 31 January 1980 to 30 January 1981.This start with a first full year is however not the rule in ancient year dating on coins, what may generate a lot of confusion.