Use of a time-clock is very acceptable as long as it is complete and accurate. Note the word "accurate". Many employees work on a fixed schedule that seldom varies at all. The employer may keep a record showing the exact schedule of daily and weekly hours and simply indicate that the worker did follow the schedule. When a worker is on a job for a longer or shorter period of time than the schedule shows, the employer must record the number of hours the worker actually worked, on an exception basis. The time you spent working over your regular 8 hours must be counted and paid as compensable hours. Adjustment thereof without your authorization is unlawful and well in disregard to the Fair Labor Standards Act (FLSA) of 1996. Below is some information that may be of further assistance.
Wage-Hour toll-free information and helpline, available 8am to 5pm in your time zone, 1-866-4USWAGE (1-866-487-9243)