Time Tracking by law

From a U.S. perspective, keeping track of employee working hours is not an optional chore: The federal Fair Labor Standards Act (FLSA), and numerous other federal and state laws, require employers to keep records of hours worked, wages paid, and other conditions of employment. Beyond the law, it is impossible to run a successful business without keeping track of employee working hours. The FLSA requires that time records show the date and time a worker's workweek starts, the number of hours worked each day, and the total hours worked during the week. For many business reasons, employers need to keep thorough, accurate records of all hours worked, including starting and quitting times for each employee. The law does not mandate just how time records are to be kept; employers have any number of options. Following are a few of them.

Time Cards
Many employers require that workers keep written track of their working hours manually on a time card or time sheet and turn them in periodically. Workers with access to computers can log in and out of work electronically.

Time Clocks and Web Clocks
Time clocks are not required by law but are often used by employers. Where they are used, employees who voluntarily clock in before their regular starting time or stay after their closing time do not have to be paid for such periods unless they are working.
Refer to this document by the US dept. of labor Facts sheet 21