In Timebutler, you do not need to enter core working hours for employees — and you cannot.
Instead, you can set the earliest start time for time tracking, either for all employees or individually per employee.
However, you cannot set a latest end time, because experience shows this restriction often causes problems in practice. If a latest end time applied and an employee happened to arrive at the workplace later than usual, they might not have enough time to meet their target working time.