Please spell out the month name. In many countries dates are formatted as DD.MM.YYYY rather than MM/DD/YYYY. For example, a native German speaker will have a lot of trouble recognizing that date. Especially if it is something like 8/7/2011 (is that August 7 or July 8?)

If you write the date as "August 8, 2011" it should be trivial to parse for everyone. (Except for people using different calendars)

Not only native Germans, other native English speakers, like people in the UK use the DD/MM/YY format. Unlike people from Germany, people from the UK have no language barrier, and are a potential customer.

Yes, you are totally right. But don't overrate the language barrier: I sell a software product that is English only (localization is too expensive), and the majority of my customers are from non-English-speaking markets.

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.

Why would you browse for what EST mean if you can simply open confirmation link and see if it works?

But the suggestions overall are valid - thanks!

What's better:

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"?

"The link will expire after 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.

"valid through August 19"

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.

Personally, I would say "August 19th", not "19".

Agreed, especially with the dropped year.

If the duration is less than three months, the year can be inferred from the date of the email.

You should drop the bit about the expiration. It requires someone to do a timezone calculation to even figure out what it means, and you probably want them to click the link even if it has expired, then you can give a proper support message with what they need to do.

Agreed on the formatting, but I bet that having that expiration date on there increases the CTR.

I think it would be even more user friendly if you included a more readable expiration. Something like "this link will expire in 2 days (8/19/2011 10:29:11 PM EST)". This gives the user a more understandable deadline, without having to check the current date

Exactly, and only, what this is: a subject with a clear call to action instead of a subject that seems to indicate that this email was generated as an auto-response to a previous action and contained a body that did not require user interaction.