You do not have to pay employees who voluntarily come in before their regular starting time or remain after their shift ends as long as they do not engage in any work (e.g., going to get coffee, talking to coworkers). If no work is performed, you may disregard their early or late clock-punching. However, if employees are punching in early or late, the employer has the burden of proving that the employee was NOT working and, therefore, need not be paid. So, don't let employees clock in until they are ready to work.
Read more about Timekeeping and Recording Guidelines in the HR Library.