Since the introduction of some new required attributes for Google Base feeds people have been having problems understanding how and when to use them. expiration_date is one of those, and if set-up wrongly could means none of your items appearing at all. 

Here is Google Base’s help documentation description of this new attribute. 

This is more a base item reference than actually when the physical product expires. Some people are seeing this as the expiration of the product like a sell by date for food. To be honest its not really clear from the description that it is. There has been people on the Google Base Group asking how they can check every products sell by date and add this data to base ! This is to tell Google Base when each product expires or the sale ends like for example eBay would use this to list all their items and when the items bidding ended. 

This is difficult to set-up if you have items especially on ebay that you list on base. Some people are making the mistake of uploading a file with dates in the past and this is making no items appear at all because by the time they are uploaded they are already expired. 

Here’s my top tip for making sure your items are never affected and are always active on google base for the longest time possible. Set the date on all your products to 2012-12-12 or a date in the far off future. Then Google Base defaults and they all expire in 30 days. So if you set them to all expire in 2012-12-12 and upload them on the 14th April 2007 then they will all default and be set to expire in 30 days, in this case 14th May 2007. 

It’s as easy as that.