I came here to get an answer to this question. I think Bryan Agee's is a fairly compelling one. Does until [date]” mean “before that date”? It's as much a legal question as it is an English question. The answer is complicated by the multiple definitions of the question -
1)Does “until [date]” mean “before that date”?
2)What does "within 2 days" mean?
3)What does "until August 18, 2011" mean?
That's really 3 questions.
Regarding the first, which is the one I came here thinking about, I feel like it's helpful to try and translate the sentence into pseudocode, and armchair evaluate it. It seems to me that conceptually, if "until (Thursday)" is evaluated every second, it is going to evaluate one way up through the last second of Wednesday, and then will evaluate the opposite way. But, pseudocode and English are not the same thing, so I'm not entirely convinced this means that the answer to #3 is is "your last possible moment to comply would be Aug 17th, 23:59:59.99.....".
Looking in a dictionary was no help; I came here after trying a few.
Regarding the second, to me, "within 2 days" means within 48 hours, so in this context, by 21:04 on August 18, 2011.
Regarding the third, I see it as like the first; the value of "until (August 18, 2011)" changes a second after Aug 17th, 23:59:59.
As for the legal definition, I found a discussion of some cases in the legal media here but it's a confusing discussion! Bottom line: in the subject case, the Illinois court judge ruled that 'until' means up to and including. (He ruled this way despite a case that the losing side claimed had set precedent the other way.) This is the opposite of the conclusion I reached via pseudocode.