You do not need to, and cannot, enter employees’ core working hours in Timebutler.
Instead, you can set the [earliest start time for time tracking(/time-tracking/settings/earliest-start-time-in-time-tracking/), both for all employees and individually per employee.
However, you cannot define a latest end time, because such a restriction often causes problems in practice. If a latest end time were set and an employee happened to arrive at the workplace later than usual, they might not have enough time to meet their required working hours.