In version SendStudio NX 1.0.4 we took out the option of allowing users to change this as it was causing many problems with the times being set wrong. If your users are located in different timezones to your server they can change this time in their user settings page. Read this knowledge base entry for more information.
To modify your server timezone in v1.0.4 and subsequent versions you will need to edit the 'admin/includes/config.php' file. Look for the line
define('SENDSTUDIO_SERVERTIMEZONE', 'GMT+11:00');
and change the GMT to what timezone you are in (in GMT).

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