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!