According to the prophecy in Daniel ch. 9, the Messiah was to appear exactly 69 weeks after the decree was made to rebuild Jerusalem.
Historical records show that Artaxerxes Longanimus made the decree for the Jews to rebuild Jerusalem in Nisan 30 445 BC. The WT states that that this happened in 455 BC.
They got 607 BC as the date of the destruction of Jerusalem wrong and now this one as well.
Although this date is not as critical as 607 BC to the WT doctrines, it still shows how they write their own history as they please. Hey at least they are consistent.