Timestamps under an /app/ page (like "Contacts", "Deals", etc) will automatically use the time zone of the computer/operating system.
The exception to this is the contact's activity stream. The activities will be displayed in the time zone set in the user account settings
Everything else will also use the time zone that the user has set up in Settings > Account. This is a per-user setting, each user can have a different time zone set up.