There is news going around here in India - That apparently GMAT is now penalising scores If one tends to take more time on a question (~2Mins) .
Has someone else heard of such cases & Maybe they are wrong as GMAC has not released any official statement - But they anyway never do.
The GMAC people I am in touch with have always denied using time taken to calculate scores (though they say that it is tracked, but that's for different reasons). You can also take the GMAT whenever you feel you are ready. You don't have to aim for a particular month.