In the "add evidence" dialog, AD Ediscovery/Summation Pro are only displaying time zone selections for the region of North America.
Enable the "show all timezones" option in "Project Defaults".
Go to Management > System Configuration > Project Defaults to change the "show all timezones" setting.
After enabling the "show all timezones" project default option, the "add evidence" dialog will display all time zones like below.
Note: the initial default timezone selection will always remain as "UTC -07:00 America/Denver".
Additional Note: Please see the "Admin Guide" for more detail about the evidence time zone setting.