We dealt with this recently. According to the MyTime help line, you can change your availability on ehr, but in order to change your max hours per week, you need to fill out a request(ask your HR team). We have to change it manually, then fax the request in to corp.
I work in HR. One of the big beefs I have with my time is that it only schedules people in their primary workplace. We were told to cross train TM's to better utilize the hours we get, but it only pulls people for the one position. This is a real pain for flow TM's who usually work truck then go...