Invalid EffectiveDate in your XML envelope. For help, see http://sellercentral.amazon.co.uk/gp/help/30641
The error refers to the EffectiveDate element in your envelope XSD. This date must be in the past to be valid, to reflect the time that your inventory was logged in your own system. It must also be in an ISO-compliant format.
February 29, 2008, 1:10:30 p.m. in Seattle should be expressed as:
We strongly recommend that you supply your Time Zone as well as the basic date information.