When the VIES return an error message, this library return the error "service returned invalid request date".
That is not correct. It should return the error message returned by VIES.

You can correct it by changing the response structure by :
var rd struct {
XMLName xml.Name xml:"Envelope"
Soap struct {
XMLName xml.Name xml:"Body"
Soap struct {
XMLName xml.Name xml:"checkVatResponse"
CountryCode string xml:"countryCode"
VATnumber string xml:"vatNumber"
RequestDate string xml:"requestDate" // 2015-03-06+01:00
Valid bool xml:"valid"
Name string xml:"name"
Address string xml:"address"
}

		SoapFault struct {
			XMLName string `xml:"Fault"`
			Code    string `xml:"faultcode"`
			Message string `xml:"faultstring"`
		}
	}
}

And check the error after the unmarshal :
if rd.Soap.SoapFault.Message != "" {
return nil, errors.New("service returned an error : " + rd.Soap.SoapFault.Message)
}

Hey, would you take contributions? As in go.mod and maybe GitHub Actions setup?