If it’s very hot outside, and you leave a (sealed, unopened, never been opened) package of air-dry clay out in the hot weather, will the clay harden?
As long as the clay has moisture it will stay soft, you may get sme surface hardening depending on how it is packed. I have been working with a lot of natural earth clays recently and air drying them, if I leave them in tupperware out side I get some surface hardening but it softens back up when I work it.