Yeah Bee said what I would have said. And these scenarios really do not relate to the original post, but the original question has been answered correctly.
But, since the issue has come up: there is a difference between changing your exit date on an overnight permit for a trip that originates at the Portal (can do so easily/freely with no restrictions beyond the 14 day thing when you pick up your permit), and changing your exit date on a permit for an overnight trip that originates elsewhere (e.g., Onion Valley). If your trip originates elsewhere (not including trips that originate in a national park, which is yet another can of worms), then the exit date that you chose when you reserved IS more fixed--you are stuck with that date when you pick up your permit, unless they happen to have excess capacity for a different date. Even the terminolgy is different on a trip like that -- you are reserving a permit for the OV TH (Kearsarge) PLUS an "exit permit" on Whitney for the specified date.
Confusing? Maybe. Draconian? No. I specifically asked the rangers about exiting early/late from a trip originating at OV and exiting WZ due to injury orO running out of food. They said they understand that stuff happens, and that so long as people are not abusing the system, they will be understanding. On my own OV-Whitney trip, while that did not happen to me, it did happen to a couple I met at Crabtree. Rob (Sequioa NP ranger) told them that the Inyo rangers would understand if they had a good reason to deviate. Later, I confirmed that yet again with Brian, an actual Inyo ranger whom I met on my way out, who was checking permits. Good reasons include injury and running out of food. They don't include abusive reasons, like, "I took this exit date only because my first choice exit date was sold out."
