A Muslim acquaintance pointed out, that Baha’is accept the Qur'an as the inerrant word of God (unlike the Bible), yet the Baha’i calendar contradicts this verse in the Qur'an, 9:36... 

The number of months with Allah has been twelve months by Allah's ordinance since the day He created the heavens and the earth. Of these four are known as forbidden [to fight in]; That is the straight usage, so do not wrong yourselves therein, and fight against the disbelievers collectively as they fight against you collectively. But know that Allah is with those who are righteous.

The pre-Islamic calendar utilized an intercalary month, a practice called Nasi', that was prohibited by Muhammad in his farewell sermon... 

Certainly the Nasi' is an impious addition, which has led the infidels into error. One year they authorize the Nasi', another year they forbid it. They observe the divine precept with respect to the number of the sacred months, but in fact they profane that which God has declared to be inviolable, and sanctify that which God has declared to be profane. Assuredly time, in its revolution, has returned to such as it was at the creation of the heavens and the earth. In the eyes of God the number of the months is twelve. Among these twelve months four are sacred, namely, Rajab, which stands alone, and three others which are consecutive.

http://bahaism.blogspot.com/