It is possible to set the default time zone and locale for an organisation using the following settings at the organisation level. All users within that organisation will inherit those settings by default.
You will need to be an organisation admin in order to access or change these settings:
It is possible to override these default settings on a per user basis by using the My Settings page accessible via the organisation drop down menu:
Using the user details dialog you can modify the Locale and Time Zone settings for the currently logged in user: