How complex is automated timetable generation?

Currently I am developing a website application, which aids in generating timetable automatically for all the classes in each department for an educational institution.In the beginning while our batch selected this topic we felt that it would be lot easier as there would be a open source for generating timetable, we can use that logic to finish the main algorithm.But as time passed we realized that it is not that simple as we thought because the pattern being used in each institution varies so it is our job now to come out with a that suits for our institution.