soup, if that's a Whitney Main Trail overnight permit, you actually have two weeks from your entry date. The 8/21 exit date is just based on what you told the Inyo staff your initial plan was. Assuming you don't have anything squirrely in your itinerary that would conflict with the regs below, you're good for up to 14 days.
From the Inyo rules and regs:
3. An overnight permit can be issued for a period of up to 14 days. To get an overnight permit you have meet the entry quota for the day you plan to enter the designated trailhead. Also, if you do not start on the main Mount Whitney trail and want to exit via the main trail, you will need to meet the exit quota for the day you expect to pass through Trail Crest in addition to the entry quota for your starting trailhead. You will not need to meet the quota for any other days on your hike. People with a Trail Crest exit date are required to pass through Trail Crest on that date, but they can exit Whitney Portal on the following day (if inside the 14-day window). For other routes, the exit date on the permit is used only for planning purposes (and you can exit any time inside the 14-day window). When you pick up your permit, you will be asked first to write down your planned overnight locations on the front of your permit, then record your actual itinerary, and finally mail back the permit to the Forest Service to help them compile usage statistics.
