We will be in Atlanta in May. We want to go to NY and back from Atlanta Hartsfield in a day. Is there internal flights we can take or can we go by car? And how long a journey is it?
Driving from Atlanta to New York City and back takes longer than 24 hours, so that's not an option (it's around 2840 km round trip).
The scheduled flight time is just over two hours, so let's call that 4.5 hours round trip. For round-trip travel between the city center and the airport, you must allow at least half an hour in Atlanta and two hours in New York, being very optimistic. You should also plan to arrive at the airport an hour before each flight. That's a total of at least nine hours travel time.
If your flight is at 6 am, you might arrive at your first destination in New York by 9 or 10. If your return flight is at 9 pm, you would want to leave for the airport by 7 at the very latest, and you could be in bed by midnight or 1 AM. It's possible, but it's a long day indeed, and bad weather or other delays could easily ruin it. I would make other plans unless the value of spending that day is extremely high for you.
This assumes that you're taking taxis to and from the airport. If you want to take public transportation, your travel time would increase by probably at least an hour on the New York City side alone, reducing the duration of your stay in the city accordingly.