See this thread
Calendar Entries- always wrong time on some computers
where KevinH concluded it was probably a problem with the "compare rules" in the Javascript code which tries to determine the TZ of the user's browser.
I haven't had a chance to do any further testing for the client who had the problem - they were running Version 3.1.1_GA_394.RHEL4 May 5, 2006.
I shall be upgrading them shortly to a later 3.1 version and shall report if the problem persists - it was a weird one!