The Timezone can be modified for the User or Business Unit, through the user interface, as showed in the images below:
In addition, you can modify it likely via WebServices.
You can use WebServices API, like:
Note that, when you change the timezone in the Business User Profile, this change will reflect for all Svc Catalog Users that belong to this BU. However, it will not reflect for the Business Unit existing Accounts.
The existing Business Unit Accounts must have the timezone modified separately, by User Interface.
Only new BU Accounts will inherith the new BU timezone.