I booked a MSP-SFO-MSP flight leaving Monday and returning Friday except in this case and without realizing it, the return was for the following Friday. It was only when I couldn't find the paper copy of my driver's license, which had its corner clipped because I had just renewed it that I found out because the woman at National that I was talking with said it was a 12 day rental. BTW, I had to get the State of MN to fax them a copy of my license and a few days later I found the original paper copy in a pocket in my backpack.
I then paid a considerable amount to rebook for that Friday on NWA and then book another flight for the following week. If that were to happen again I would have just booked a second flight SFO-MSP-SFO for the weekend.