I had a conversion at the visitor's center about this very matter a few years ago. Their position is that your day hike permit is for the main Mt Whitney Trail. The trail starts at the Portal. Therefore, you cannot leave the Portal until midnight.
There is a popular misconception that you are OK if you don't pass the Whitney Zone sign until midnight. That is not correct. Hikers are confused because they can hike to Lone Pine Lake anytime. If you were at Lone Pine Lake at midnight with a day hike permit, you are expected to hike back to the Portal and begin your Mt. Whitney hike there.
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Do you have any reference to a document confirming this?
I do not question your statement that one of the rangers told you this. Is it possible that this ranger has been mistaken?
BTW, have you had this conversation on April 1st?
