I booked my rental for the correct day, wrong month. Pre-paid through Kayak. They laughed with us and said not to worry about it - they had a car and it was fine. Didn't charge us a thing, gave us great driving advice (red always means stop, for example) and gave us a discounted way to cross bridges. And the car was new, clean, and fun to drive.
What's not to like? Best car rental experience of my life!