I've always found "through" to be confusing when used like this. It's like the whole "Next Friday" thing - is that the one about to happen (it's the "next" one to occur), or is it the one after that? (in Australia, it's usually the later)
I much prefer "valid until August 19"
(I'm a native English speaker, not from the US where "through" seems more common)
(I'm a native English speaker, not from the US where "through" seems more common)