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.