is this illegal at work regarding pay

At my current place of employment at this ltc/nursing home, u clock in and out when ur done with ur work but thing is, even if u stay extra u do not get paid for it. it is required that u have a really good excuse for staying late and u need to have it approved by the supervisor in order to get the pay for it. otherwise u can stay 1 or 2 hr late bc ur not done with ur medicare charting or what not (which is required each shift) and not get paid for it. even if u tell the superviser bc u have admissions or what not, its still not a valid excuse. however, if u clock in late, it will show what time u clock in on ur paystub but even if u clock out late, it will show that u leave exactly at 11:30 (i work 3-11 shift). so i usually stay a few min late when i come to work late to make up for it because it shows on my paystub but when i leave late it always say i leave on time and come on time. is this illegal? the only way someone gets ot in this facility is to work an extra day or shift. what should i do? if this is illegal, why has this been going on for years and years? or is this how most ltc and nursing homes run by?