You can set the default time zone for your account as well as for individual domains.  Users can also overide the default timezone for their own login.

Change Account level time zone

1. Navigate to Account > Account contacts
2. Click on the link near the top of the page following Default timezone for account.

Change Domain level time zone

1. Navigate to Settings > Quarantine users
2. Click on the link near the top of the page following Timezone

Change User level time zone

1. Navigate to Account > Account contacts
2. Click on the row containing the user which you'd like to change the timezone for
3. Select a new time zone via the list box
4. Click on the Save button