The JavaScript engines in Firefox 2 (Windows) and IE6 can't handle the new Daylight Savings Time rules in the U.S. The Date() function returns a value that is off by an hour if the system time is between the second Sunday of March and the first Sunday of April.

More at the Cozi Tech Blog.

Update 2008/03/14: Mea culpa. This is not a widespread problem. It is caused by the presence of set TZ=PST8PDT in my C:\AutoExec.bat. Per KB932590, the existence of the TZ environment variable will cause the CRT to use the old DST rules. (I can't remember why I set TZ several years ago. It's part of the accumulated mess of files that I bring everywhere with me.)