2100 is fine, that is late enough as the majority of trips end in ATL at or before 2000. Where most people have issues is when their last flight is at 1945 for example and their day four is delayed due to weather/mx etc and they don't have that 2100-2300 late departure back home as a fail safe. Worst case scenario, sleep in the lounge on day 4 if you're done very late and fly out of ATL at 0730 the next morning to wherever you live. Sucks, but inevitably it will happen at least once. Welcome aboard, did you just get hired?