Me and a fellow employee have noticed anomolies on our time records at the Minnesota located gas station where we work. Basically the time clocked seems to shrink mid week. We are currently logging our time independently without the manager knowing so we have some evidence to present. What is the next step in evidence gathering/proceeding and what are the limits of an employer altering your time system?

Info about time system: Computer based, select login and enter pass. As far as I know only assistant managers and higher have permissions to overide times.