Esther 3:7 KJV - "In the first month, that is, the month Nisan, in the twelfth year of king Ahasuerus, they cast Pur, that is, the lot, before Haman from day to day, and from month to month, to the twelfth month, that is, the month Adar."

To me, the above verse becomes more clear upon realizing that it includes three explanatory remarks. As follows:

Esther 3:7 - "In the first month [that is, the month Nisan, in the twelfth year of king Ahasuerus,] they cast Pur [that is, the lot,] before Haman from day to day, and from month to month, to the twelfth month [that is, the month Adar.]"

Realizing also that the explanatory remarks in Esther are intended to give reference to the Babylonian or Persian calendar (that is, in contradistinction to the default Scriptural calendar) this helps me make sense out the events recorded within the Book of Esther. Said 1st month of the 12th Babylonian or Persian year corresponds to month one of the 11th Scripture year, which fell in 474 BCE. It follows that the 12th month of the 12th Babylonian or Persian year corresponds to the 12th month of the 12th Scripture year, which fell in 473 BCE.

If the reference provided in Esther 3:7 had been to the Scripture calendar (to the 11th year) re the events of the 1st month, then an additional reference re the events of the 12th month would have been required also (to the 12th year.) However, no such additional reference is provided.

I should haste to add also that it is a moot point whether or not the explanatory remarks within Esther were written by the original author or by a later transcriber of the text. In fact, considering the importance of those explanatory remarks for the integrity of the dated events within Esther, I believe it is most likely, if not certain, that they were all written by the original author.