Surely you don't need expiry to be accurate to the second. Also, most people in the world have no idea where "EST" is (or even what it means).
Why not just say "The link is valid until August 19", and then let it expire some time after all the world has moved on to the 20th? (No MM/DD or DD/MM. Please.)
I personally prefer the old fashioned "Just reply to this mail" method, which many mailing lists use for confirmation, to opening a browser window - in the worst case I have to wait for my browser to launch, too - and I'm going to close that window immediately anyway so I can get back to what I was actually doing, so I just think "I'll get to that later", but then I don't, and when I remember [did I click that link? I'm pretty sure I did, so there's probably a bug in their list somewhere and I shall ignore them henceforth], the link has probably expired anyway [can't be bothered checking what time and day it is in "EST" (Estonia?)] so I'll just browse around for funny kittens instead. And all of that just because somebody wanted me to click on a link. They're sooo cute!
Maybe that's just me, but everytime I see a "just reply to this email", I stop and wonder about the details. Do I have to put something more in the body ? Will my mail go through ? Is it automated, or will a real person on the other side process my answer ?
Maybe it's just that the email way is too rarely used, but it makes me pause. I prefer a link, that's much more of a no-brainer.
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)
Right. "Through" is better than "until" because "until" leaves the reader unsure whether it includes August 19th. And 2011 is unnecessary; if someone does find an old message, they can look at the Date: header.