Shelf life is 6-7 years so having them is actually pretty smart, Japan or not.
In reality, they do not lose efficacy over time:
"Yes, potassium iodide tablets are inherently stable and do not lose their effectiveness over time. Manufacturers must label their products with a shelf-life to ensure that consumers purchase safe and useful products.
According to FDA guidance on Shelf-life Extension , studies over many years have confirmed that none of the components of KI tablets, including the active ingredient, has any significant potential for chemical degradation or interaction with other components or with components of the container closure system when stored according to labeled directions. To date, the only observed changes during stability (shelf-life) testing have been the failure of some batches of KI tablets to meet dissolution specifications. Some tablets tested required slightly longer than the specified time to achieve dissolution. Even in the case of a failure of this sort, the product remains usable. In such cases, instructions can be provided to crush the tablets and mix them with a juice or other liquid prior to administration as suggested for emergency pediatric dosing."
I don't know when I got them. Someone delivered them to my door during the TMI thing. Looking at it as I type. It is about 11-12 miles away.
It says IOSAT, Expiration 8/2007.
Not sure but I don't think they go bad, just lose their strength. Not sure.