Maybe you should ask them for $100 a day for the extension. You can back out anytime they ask for an extension don't let them give you any guff, they must refund your deposit, they are the ones breaking the contract. In CA you have 3 days anytime there is a change to back out.
That said, a lot of escrows close late, it may be that the house they are buying is the problem. Try to keep it friendly; angry people do a lot of crappy things. Communication is the key. Find out if the real reason they are not closing is a propblem that may not be curable such as a lien on title.