I just purchased a new cookbook. There is a recipe in it for Cream Fraiche.
It calls for 3 cups of heavy cream and 1 cup sour cream. It says to mix the heavy cream with the sour cream, place in a glass bowl covered with plastic wrap and let it sit at room tep for 24 hours. After 24 hours remix and place in the refigerator for up to 2 weeks.
Won't this sour, go bad if you leave it out for 24 hours?

Nope. You should be fine. There is good bacteria cultures present that keep it from going bad, and is why it is considered safe. Use pasteurized heavy cream not ultra pasteurized. After the 24hrs the mixture should have thickened up. Creme fraiche by nature has a sour taste but not as sour as sour cream. Hope this helps.