You are changing time zones when you travel from Atlanta to LA and then back. Most nonstop flights between the two are actually about 5 hours, give or take a little (usually shorter on the way back, actually). So you may have a layover if your travel time is significantly more than that.
But if you leave from Atlanta at 12 noon, you will arrive in LA around 5:00pm Atlanta-time/EST, which is 2pm in LA/PST. Likewise, if you leave LA at 12 noon, you will arrive in Atlanta around 4-5:00pm LA-time/PST, which will be 7-8:00pm in Atlanta/EST Flight times on your itinerary are always local. So it might look like you only have a 2-2.5 hour flight there, and a 7-hr flight back, but the actual flight times are comparable. Flight times are usually provided on your itinerary.