Is this due date right for the time of conception?

Is it possible to have during the week of December 22- Jan 2 2008 and have a due date of Sept 4 2009. How does this work it seems as if the due date would be somewhere around sept 12-20 2009, right. Please help me figure thi out how do you justify having during this time and ending up with a sept. 4 due date??????? Last period was Nov. 27-28 2008