The יערות דבש explains in ח"ב דרוש ב that the :גמרא מגילה יב refers to חז"ל as יודעי העיתים in מגילת אסתר because they knew how to intercalate the months of the year and set the months-- שיודעין לעבר השנה ולקבוע חדשים-- because אחשורוש was מסופק whether ושתי was 20 years old or not, but he proves that she was surely 19 years and twelve months old, the שאילה was whether or not that year was מעוברת-- pregnant, meaning had thirteen months or twelve months. He writes, based on the words of מדרש שוחר טוב על תהלים that people are not obligated for punishments until they reach age of twenty, so אחשורוש had to ask the רבנן whether or not he should execute ושתי for refusing to attend his משתה. However, I had a קשיא on this, because all ages in the הלכה are really הלל"מ like any type of שיעורים במצוות, so if they are only הלל"מ then they don't apply to non-Jews like the רמב"ם himself says. So then what was אחשורוש's problem? The same question applies to the בן יהוידע there in מגילה. Therefore, I wrote on the side of my גמ', the following, "צ"ע".
However, now I see another פשט from הרב יהונתן איבשייץ זצלל"ה:
He explains why it seems Achashveirosh made 2 parties. One in the 3rd year of his reign and another one 180 days later. He writes that Achashveirosh was counting 70 years until the golus was over and when the geulah didn't come he made a party. However, Achashveirosh wasn't sure if the years followed the Malchei Yehuda which is from Nisan or the non Jewish kings which is from Tishrei. When Tishrei of the 70th year came he started his party., however he was still wary that teh geulah would come in Nisan. 180 days later when Nisan came and there was no geulah, he figured it would never come. He got bolder and made a second party andthis time told Vashti to drop everything (pun intended) and come to the party.
One final point, Achashveirosh was still mesupak if Adar would be m'ubar. Therefore he asked the "yodei ha'ittim" who were the Sanhedrin who decided when to make an extra month.