Is it legal for a manager to change clock in/out times of an employee?

There is a concept called rounding up and rounding down in wage and hour rules that allows an employer to "change" your actual times clock times -- or at least pay you for time that doesn't exactly match up with your clock times but it has to work according to set rules and the employer has no "say so" on whether you are rounded up or down for a particular day.  If your employer is regualry changing your clock times to show that you work only 40 hours in a week, that's probably a violation of the Fair labor Standards Act. PA wage payment and collection law and you should be entitled to back pay plus statutory interest and penalties.  You should raise this matter with your employer or retain an attorney to get involved on your behalf.