Problem: October 31st at around 7PM CST I logged in to finish the last few things I needed to finish my monthly achievement. Unfortunately, I forgot to compensate for the fact that the game seems to operate on GMT for the purposes of daily and monthly achievements. I had spent a LOT of time and energy working on that achievement and as far as I was concerned, I had 5 more hours to finish it up. This was EXTREMELY irritating.
Solution: Have the monthly achievements clear and start new 24 hours later. (ie Midnight between the 1st and 2nd day of the month GMT instead of between the last day of the month and the first day of the month GMT) This would prevent any misunderstandings due to the time changes. People will be less irritated at having one less day at the beginning of the month to work on an achievement rather than being cut short at the end of the month.