You can stay as many extra nights as you need (there is only a two-week limit on staying in one place). The permit you get only requires (and restricts) you to enter on the entry date shown on the permit.
If you are trying to change a DAY hike permit into an overnight, that is a different problem. There will likely be no-show permits available when you show up to pick up your permit, but the parking permits are necessary after July 10, and switching day permit to overnight may be tough.
So, we have an entry date of July 6th and it's an overnight permit. We don't think we have enough time to summit, hike down, and drive back on the 7th to be back to work on the 8th. We can't, unfortunately, take the day off work on the 8th.
I've done Mt. Shasta, but not Whitney and Whitney seems like an easier trek, but my wife hasn't gone above 8k feet. So, we will need to take it slow.
I'm wondering if there's a possibility to hike in on the 5th (When we pick up our permit) instead of the 6th. Is that most likely a possibility?
