The easiest solution is that Nehemiah was counting the Jewish way - Tishri to Tishri (Babylonians and Persians counted kings' reigns from Nisan to Nisan). So the Nisan in Neh. 2:1 didn't ring in a new regnal year (21st year) by Nehemiah's reckoning, but stayed within the Tishri-Tishri 20th year. This is the conclusion of some respected Bible scholars and chronologists, e.g. Edwin R. Thiele.
"Historical evidence points to 475 B.C.E. as the year of Artaxerxes’ ascension to the throne." [w06 2/1 p.8]
That's incorrect. Historical evidence points overwhelmingly to 465 BCE as Artaxerxes I's accession year. But that's a whole other discussion in itself.
JW Insider reacted to Ann O'Maly in Nehemiah 1:1 vs. 2:1
