You can change the timezone in which dates and times are displayed in the Account Portal Logs section. Your timezone selection will not affect how all dates and times are displayed in the Usage and Billing section; these sections will be displayed in UTC. Changing the timezone in your account portal will not affect your API responses; all dates returned in API responses are given in RFC 2822 format.
To change your timezone, visit the User Settings page. If your account has multiple users, each of them may select their own timezone by navigating to their own User Settings page.