Best of Luck,
The offer is a legal and binding document.
My suggestion would be for your agent and or attorney to contact the other parties and see if it is acceptable to them.
Again as everyone stated below it is not uncommon to make the change, but reasonable and the authority granted can be completely different.
Remember the buyer has obligations and time constraints on them as well. Hopefully they have nothing to sell,are reasonable and love your home and like you.
The standard Massachusetts Purchase and Sale is pretty clear on that, if that is what you used. If it becomes contentious is is time for you to use an attorney. Many times a buyer has financing to square away, movers have been scheduled and if the date is pushed farther out they will not have a place to live. This may become a financial burden for you the buyer.