"Will you please get this done by the 15th instead of the 20th"
"We agreed on the 20th. What has changed to make this more urgent?" Knowing the client's context might allow you to propose a better solution.
If there is NO chance that the project can be done earlier, don't give them any false hope. That will only make things worse down the road.
Use this discussion as an opportunity to figure out what is important vs urgent. Find a way to turn this request into more business by way of an extended engagement or higher per-hour fees.