Product Expiration Attribute
This is the date that an item listing will expire. Note: The default expiration time is 30 days if the attribute is not provided. But as the feed expires after 30 days anyway, this means the feed must be renewed sooner than this, which means that it shouldnt ever go out of date.

You can modify the date for an individual product in the Joomla Google Base Component, but dont forget you changed it, for the date is set, so it will keep that date even in the future, so future submissions of the feed, will have out of date products and Google will drop those products.

<g:expiration_date>

2004-08-19

</g:expiration_date>

The component has a calendar link that you click on, select a date and it prefills the date in the correct format, which is added in the correct format as above in the sample.