Incorrect certificate information may be displayed when you install a sidebar gadget on a Windows Vista-based computer: “This certificate has expired or is not yet valid”
When you try to install a gadget on a Windows Vista-based computer, you are prompted with a Windows Sidebar – Security Warning dialog box that contains the publisher information for the gadget. When you click the publisher name and view the certificate information, you may see the following message even though the time stamp indicates that the certificate has not expired:
This certificate has expired or is not yet valid.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB969808.