How can I set the default timezone?
By default, dates and times will default to the logged in user's timezone as set on their computer.
This can be set by your CSM or Vanilla support, simply select timezone from this php timezone support list: http://www.php.net/manual/en/timezones.php and email your CSM or support with your request.
How can I customize the timestamps in Vanilla?
Timestamps cannot be customized in Vanilla, but they will render based on the end user's browser language automatically.