When ZCS is installed, the time zone is configured and set in the default COS. This time zone should be the time zone that the majority of users in the COS will be located in. If users are distributed such that there is no common time zone then you will need to set, or have the users set, the time zone for each account.
To change the time zone either in the COS or for Accounts go the Preferences tab and in the Calendar Options section, select the appropriate time zone.