Both parties don't have any confidence in each other right now. A strictly enforced timeline will assure that the Palestinians will have to meet their secu- rity obligations towards Israel and on the other hand it will guarantee the Palestinians the foundation of their own state within the next 18 months. This is exactly the "stuff" that's needed in order to build trust on both sides.
High Court: Illegal West Bank homes to be demolished (Haaretz)
from the article: Israel, PA agree to strive for deal by end of 2008