Whatever you call Merchant Center/Shopping/Base, the expiry date format has changed at Google's end with no warning.
Dates need to have the correct number of digits for month and day.
So 2011-6-28 is now a failure and 2011-06-28 should be accepted, though I haven't tried it without the time on the end.
It recommends that you add "THH:MM:SS".
So what I have tried is 2011-06-28T23:59:59 and that format is accepted.