I think the expression is "turn into a pumpkin" (obviously a reference to Cinderella's carriage magically turning back into a pumpkin at midnight) as if it's an all or nothing proposition. The physical properties of something don't just magically go bad as soon as a time or date is reached.
A lot of toxins are still virtually as potent as they were when first packaged. They're actually quite stable, but industry standard is to include an expiration date for many products.