Can I roast a chicken over 2 time periods?

Can it be done? yes.

Is there a higher risk of food poisoning? yes.

The way to get around that is almost more of a pain -- you cook it in a low oven until it's just barely cooked through (which is a different temperature for the legs vs. the breast meat), then chill it, and then return it to a hot oven to warm it and crisp the skin up.

You're often better off fully cooking it, chilling it, and then warming it back up, and serving with a hot gravy that you can pour over it.

Another alternative would be spatchcocking the chicken (cutting the backbone out and flattening it, so it cooks quicker; see https://www.seriouseats.com/butterflied-roasted-chicken-with-quick-jus-recipe ). You could prep it in advance, so you just had to throw it in the oven when you got home ... but you have to cook it under the broiler for a chance at the 30 minute window: https://www.foodnetwork.com/recipes/alton-brown/broiled-butterflied-chicken-recipe-1951266 )

There's also a technique where you quarter the chicken, start it on the stovetop (starting the leg quarters first), then move it to the oven that's been pre-heating ... and if you had the chicken prepped might take 30 minutes ... but I'm having difficulty finding the recipe. (I thought it had been on Good Eats, but I'm not having luck searching the Food Network website).

When I have wanted to cook a chicken but I will not be home, I have used a slow cooker. You won't end up with a really crispy skin, but it makes for some wonderfully moist and juicy chicken.

Most slow cookers can be set for anywhere from 4 to 10 hours, so you should be able to find something that fits your schedule.

As others have elaborated, cooking in two steps will lead to food safety issues if you don’t make sure that your food is cooled down properly in between. And yes, that also has the side effect of likely running into issues with drying out and overcooking at least in some parts.

Let me offer an alternative idea to combine your timing constraints with a properly cooked chicken: slow roasting.

If you put your bird in a ~300° F oven, you have about three hours until it’s done. That should be enough for a visit to the playground plus finishing the sides when you get home. Timing etc. will of course depend on size of the bird etc., but I recommend you do a bit of research into that method. A meat thermometer will help you determine when it’s done to a safe core temperature.

How about cooking it sous vide?

This method involves some prep work, but gives you the timing you want. Essentially, it consists of sealing the chicken pieces in plastic, and immersing them in water with apparatus that heats the water to a safe temperature for cooking without drying it out. You can then finish on a hot pan or in a hot oven to get crispy skin.

You can chill the chicken after the sous vide phase, allowing you to do that part of the cooking on a different day from the finishing.

As said it can be done, but i wouldn't do it. Especially on chicken is it important that it is 'well done', if you put it in the oven it needs time to get warm inside. So if you stop heating it, you will heat the outside 'twice' but you still need a lot of time to heat the inside on the necessary temperature. So cooking don't get much faster, but the chicken gets dry outside. You should considering it to put it in the oven with a fitting temperature, so that it is finished when you are back (low temperature) or finish the chicken and then just heating it when you want to eat.