Megillah 11 - Computational mistake

As predicted by Jeremiah, Jews would be redeemed from Babylon and returned to Israel after seventy years. Now, Ahashverosh made this claim: "My predecessor, Belshazzar, made the calculation but made a mistake; however, I will not make a mistake." The Talmud describes Belshazzar's calculation, how he celebrated, and how he got punished.

Ahashverosh sought to correct the previous mistake, and based on the analysis of the writing, he started counting seventy years from a different historical event. However, he celebrated in the same manner by using the Jerusalem Temple's vessels at the banquet. This started the chain of events that the Megillah describes, including the killing of Ahashverosh's wife, Vashti, who was the daughter of Belshazzar.

But Ahashveroshes's calculations seemed correct!? - He should have started from the year of the destruction of the Temple. 

Art: Frans Francken Belshazzar Feast