For each Matter a default Time Zone is selected. This selection sets the time zone offset that appears for times listed in the grid screen and appears on the native/images of all data processed. Changing this setting (Matter - Properties) modifies the default setting on all future processed data and does not modify any data already processed.
When processing each collection the default is applied, however, a drop down is available to select a specific time zone for a specific import.
To modify a time zone after processing click on the Import listed in each collection and select from the drop down. 
Private Cloud:
For Digital WarRoom Private Cloud customers the operational Time Zone of each matter will be the Time Zone setting of the server. The setting of that Time Zone affects operational dates and times such as when a mark or issue was applied.
Single Matter:
For Digital WarRoom Single Matter the operational Time Zone will always be the Central Time Zone as that is where the servers for those matter reside.