In All Hours, the time zone is set for each calendar group. All employees who are assigned the same calendar group use its time zone, so employees working in different time zones need different calendar groups.
Changing the time zone
Go to Settings › Calendar Groups and open the calendar group assigned to the employees.
Click Edit.
Select the Time zone. In the same window you can also change the calendar name and workdays.
Click Save.
The current time zone of each calendar group is shown in the Time zone column of the list and at the top of the calendar group page.
Employees in another time zone
Create a new calendar group with the correct time zone and holidays (Creating a new calendar) and assign it to these employees: in Users, select the employees, click Actions and choose Assign calendar group.