Employer changing clock in/out times of workers to lessen hours and pay

I really dont know what flair this fits into. At my job we have to clock in/out every day at a computer as usual. Every day is a different start time and if you are late you get reprimanded, so the supervisors tell us to arrive 10 minutes early. I never arrive late and keep track of my times. When I look at my paycheck and time card for the week, I see that the times have been altered to later clock ins and earlier clock outs, saying I worked less than I actually did (so I get paid less). It’s my first job so I don’t know if other jobs experience this but everyone complains about it at my work. There is no legal justification to this right? It’s union if that helps.

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