type Timestamp time.Time
func (t *Timestamp) MarshalXMLAttr(name xml.Name) (xml.Attr, error) {
    return xml.Attr{Name: name, Value: t.time2str()}, nil
}

func (t *Timestamp) UnmarshalXMLAttr(attr xml.Attr) (err error) {
    return t.str2time(attr.Value)
}

type MyStruct struct {
    Start            Timestamp      `xml:"start,attr"`
}

import MyPkg "github.com/user/pkg"
...

var MyStructDefinition MyPkg.MyStruct

MyStructDefinition.Timestamp = MyPkg.Timestamp{}

marshalled, err := xml.Marshal(run)
if err != nil {
    fmt.Println(err)
}

xml: unsupported type: MyPkg.Timestamp