Webtrends Analytics 9 Adminstration Guide, November 2012
If you want to generate session cookies rather than persistent cookies, you remove the expiration date parameter from the cookie.
Removing the expiration data parameter from the cookie is not recommended because Webtrends cannot use session cookies to accurately track unique visitors to your site. Also, visit counts are inaccurate if the visitor closes the browser, reopens it, and immediately returns to your site. In this case, the visitor is identified as a new unique visitor, and the visit is considered a new visit to your site.
var expiry="; expires="+dExp.toGMTString();
so that it looks like this:
Since session cookies are only valid for the current visit, they cannot be used to accurately report on many aspects of visitor data, including unique visitors, campaign tracking, commerce tracking, search engine history, and other visitor history based analysis reports. Because the use of session cookies may alter the statistics in reports from what you are used to viewing, you should try session cookies on one profile as a test model and look at the numbers in the resulting report to see if that is what you were expecting. After you accept the results, you can apply session cookies to other profiles.