In dating systems, zero year (LH:4) refers to the base year or year around which a dating system is structured, as in "years before", e.g. BC or BE, this base year, and "years after", e.g. AD or AE, the base year, the "base year" itself defined as year "zero", with years before and after counted as natural numbers, backward and forward, respectively.
The Dionysian calendar (BC/AC), invented in 525AD, has no "zero year", as the zero concept, supposedly, was not yet realized, colloquially, at this period of time. Hence, the year of the birth of Christ is counted as 1AD.
In the Thimsian calendar (BE/AE), invented in 2020AD (65AE), the "zero year" is defined as 1955AD, the year when Erwin Muller saw atoms with his own eyes. Hence, 1955AD is 0AE, 1956AD is 1AE, 1957 is 2AE, etc., and 1954AD is 1BE, 1953AD is 2BE, etc.
The following are related quotes:
- “ Herod died in 3BC, years before the supposed birth of Christ. Dionysius was clearly wrong; today most scholars believe that the birth of Christ was in 4 BC. Dionysius was a few years off. In truth, this mistake was not so terrible. When choosing the first year of a calendar, it really doesn't matter which year is chosen, so long as everything is consistent after that. A four-year error is inconsequential if everyone agrees to make the same mistake, as, indeed, we have. But there was a more serious problem with Dionysius' calendar: zero. There was no ‘year zero’. Normally this would be no big deal; most calendars of that day started with the year one, not the year zero. Dionysius didn't even have a choice; he didn't know about zero. He was brought up after the decline of the Roman Empire. Even during the heyday of Rome, the Romans were not exactly math whizzes. In the year 525, at the start of the Dark Ages, all Westerners clung to the clunky Roman style of numbers, and there was no ‘zero’ in that counting system. To Dionysius, the first year of Our Lord was, naturally, the year I. The next year was year II, and Dionysius came to this conclusion in the year DXXV.”
- — Charles Deife (2019), Zero: the Biography of a Dangerous Idea (pg. #)
- Seife, Charles. (2019). Zero: the Biography of a Dangerous Idea (pg. #). Souvenir.