Hi,
When you are authenticated it uses the time zone preference from your user profile. For unauthenticated visitors it uses the config setting
<add key="PreferredGreenwichMeantimeOffset" value="-4.00" />, you should set this to your preferred default timezone offset, ideally you should put this in user.config instead of Web.config to avoid losing the setting during upgrades.
For more info http://www.mojoportal.com/timezonelocalization.aspx
Hope it helps,
Joe