Any XML document can contain only one root element. The YML format uses the <yml_catalog> element as the root element. The "date" attribute for the <yml_catalog> element must match the date and time the YML file was generated on the store side. The date must have the format YYYY-MM-DD hh:mm.
<yml_catalog date="2010-04-01 17:00"> <shop> ... </shop> </yml_catalog>