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 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.
But the suggestions overall are valid - thanks!
1) "The link will expire 2011 August 19"
2) "The link is valid until 2011 August 19"
3) "The link is valid for one day until 2011 August 19"?
This gives people a sense of urgency, but doesn't discourage them from trying the link if they're a little late. "Through" is (to my ears) an Americanism.
(I'm a native English speaker, not from the US where "through" seems more common)
Personally, I would say "August 19th", not "19".
If the duration is less than three months, the year can be inferred from the date of the email.