There are no legal guidelines that determine what hours employees' work days include. Those hours are determined by the company. However, in general, a work day is defined as the period between the time on any particular day when such employee starts work related activities and the time on that day he or she stop work related activities. So, the actual workday may be longer than the employee's scheduled shift if they have to get changed into a uniform or perform other activities.
There are no limits to the number of hours adults (over 18) can work in day. However, minors (under age 18) are limited to the number of hours they can work. Those limits are determined by child labor law restrictions.