How accurate it was would be interesting to know, I haven’t heard about that.
However, accurate or not, it is an oopart for its time for sure.
https://en.wikipedia.org/wiki/Antikythera_mechanism#Accuracy
Investigations by Freeth and Jones reveal that their simulated mechanism is not particularly accurate, the Mars pointer being up to 38° off at times. This is not due to inaccuracies in gearing ratios in the mechanism, but rather to inadequacies in the Greek theory. The accuracy could not have been improved until first Ptolemy put forth his Planetary Hypotheses in the second half of the second century AD and then the introduction of Kepler’s Second Law.[6]
In short, the Antikythera Mechanism was a machine designed to predict celestial phenomena according to the sophisticated astronomical theories current in its day, the sole witness to a lost history of brilliant engineering, a conception of pure genius, one of the great wonders of the ancient world—but it didn’t really work very well![6]
In addition to theoretical accuracy, there is the matter of mechanical accuracy. Freeth and Jones note that the inevitable “looseness” in the mechanism due to the hand-built gears, with their triangular teeth and the frictions between gears, and in bearing surfaces, probably would have swamped the finer solar and lunar correction mechanisms built into it:
Though the engineering was remarkable for its era, recent research indicates that its design conception exceeded the engineering precision of its manufacture by a wide margin—with considerable accumulative inaccuracies in the gear trains, which would have cancelled out many of the subtle anomalies built into its design.