The default time zone for your Dynamic Signal community is a system-wide setting and is set at the time your community is created. Because changing the time zone can affect parts of the platform, such as reporting and scheduled actions, it is highly discouraged to change this, and can only be changed by Dynamic Signal.
If you wish to change the timezone for the entire community, please reach out to our Support Team or your CSM so that they can make you aware of what may be affected by the change.