The 20th year of Artaxerxes I was 455 BC
According to the Cambridge Ancient History, King Darius I of Persia died in 485 BC, King Xerxes his son, acceded in that year and so his first regnal year was 484 BC, so his 20th was 465. In this year in August, he was murdered by his son (who was not Artaxerxes) according to a Babylonian eclipse text (CAH V p13). So 465 is taken as his successor Artaxerxes' accession year and 464 as his first regnal year. But the Watchtower, have produced very good evidence to our thinking that Xerxes was coregent with his father Darius I for 10 years (Insight book II - Persia).
Everybody agrees that the first regnal year of Darius II began in 423Nisan (March/April), with Artaxerxes I dying in the winter of the Hebrew year from 424Nisan to 424Adar. Now the problem is that in the British Museum there is a tablet BM65494 which is a business document from Borsippa that is dated to the 50th year of Artaxerxes I. The next problem is that the Babylonian Expedition of the University of Pennsylvania found a tablet connecting the last regnal year of Artaxerxes I with the accession year of Darius II. It says:
51st year, accession year, 12th month, day 20, Darius, king of lands (CBM 12803, Cuneiform Texts, Vol VIII Part I by Albert T. Clay)
But we know that this year was 424 BC. So this would mean that the first regnal year of Artaxerxes I was 474 BC, 10 years earlier than the figure of 464 BC proposed by CAH. This would mean that Artaxerxes I acceded in 475 BC, which being the 20th year of Xerxes, would mean that Xerxes acceded to full coregency in 495 BC and had 494 BC as his first coregnal year, rather than 484 as proposed by CAH.
It does not seem possible to us that both the tablet in the British Museum and the one found by the University of Pennsylvania can be forgeries or errors. Therefore we must deduce that Artaxerxes I did indeed accede in 475 BC. So his 20th year would have been 455 BC.
If the 20th year was indeed 455, then the period of time from the inauguration of the Tabernacle in the wilderness in 1512Nisan to the inauguration of Solomon's temple in 1026Tishri, a period of 486½ years, precisely prefigures in time the period between the inauguration of clean worship in Zerubbabel's temple in 455Tishri, at the re-establishment of the festival of Booths and the oath to go back under law in relation to this temple:
Accordingly Ezra the priest brought the law before the congregation of men as well as of women and of all intelligent enough to listen, on the first day of the 7th month (Nehemiah 8:2).
Then they found written in the law that Jehovah had commanded by means of Moses that the sons of Israel should dwell in booths during the festival in the seventh month (Nehemiah 8:14).
Thus all the congregation of those who had come back from the captivity made booths and took up dwelling in the booths; for the sons of Israel had not done that way from the days of Joshua the son of Nun until that day, so that there came to be very great rejoicing.
And there was a reading aloud of the book of the law of the [true] God day by day, from the first day until the last day; and they went on holding the festival 7 days, and on the 8th day there was a solemn assembly, according to the rule (Nehemiah 8:17,18).
As for the rest of the people, the priests, the Levites, the gatekeepers, the singers, the Nethinim and everyone separating himself from the peoples of the lands to the law of the [true] God, their wives, their sons and their daughters, everyone having knowledge [and] understanding, they were adhering to their brothers, their majestic ones, and coming into [liability to] a curse and into an oath, to walk in the law of the [true] God, which had been given by the hand of Moses the servant of the [true] God, and to keep and to perform all the commandments of Jehovah our Lord and his judicial decisions and his regulations (Nehemiah 10:28,29).
... and the inauguration of the temple of Jesus' body in 33Nisan, a further period of 486½ years. This obvious parallel is strong evidence that the 455 BC date for the 20th year of Artaxerxes I is correct. Graphically this looks like:
Inauguration of the tabernacle Inauguration of Solomon's temple
Inauguration of Zerubbabel's temple under law Inauguration of the temple of Jesus' body (FDS1)