* Corrected on Feb 24.

Many of you super JavaScript programmers already know this, but since I didn't have it quite right I thought I'd post it here so that if someone else is searching for information they'll have an easier time of it.

When you set a cookie in JavaScript, you make it 'permanent' (make it last through browser restarts) by setting a parameter called "expires". There are web pages all over the place that will tell you about lots of different formats you can set that expiration date in. Many of them work for Mozilla and other other browsers.

For Internet Explorer -- particularly for Internet Explorer version 7 -- there is only one format that actually works in a reliable way. There are technotes that claim you can use "14 day" or "1 year" or "2 hours". There are technotes out there that claim you can use a unix numeric time value (the number of seconds since 1/1/1970). While these may work for other browsers, you will waste your time if you try them with IE7. Worse, it will not give you any error or indication that it failed. It simply won't keep the cookie if you restart the browser.

Yes, sometimes other formats work. Not reliably however. The "Correct" format* according to Microsoft is:

DAY, DD-MON-YYYY HH:MM:SS GMT

The values are as follows:

DAY = Mon, Tue, Wed, Thu, Fri, Sat, Sun
MON = A Three Letter Version of the Month (Jan, Feb, ...)
DD = A two digit number representing the Day
YYYY = A four digit number representing the Year
HH = A two digit number representing the hour, in 24 hour time format.
MM = A two digit number representing the Minute
SS = A two digit number representing the Second
GMT = Always set dates in GMT time, and include GMT on the string

For all numeric values, you MUST use both digits. Pad with a leading zero as needed

* I have noted that if you tell Microsoft's JavaScript implementation to output Date.toGMTString() you get this in the same format but without dashes between the date components:

DAY, DD-MON-YYYY HH:MM:SS GMT

Since the common JavaScript way to do this is to use a data object this way, it is safe to assume this is also supported. In my testing, it does work.