golang custom datetime type

type customTime struct {
    time.Time
}
func (t *customTime) UnmarshalJSON(b []byte) (err error) {
    layout := []string{
        "2006-01-02 15:04:05 MST",
        "2006-01-02 15:04:05 Z07:00",
        "2006-01-02 15:04:05 Z0700",
        time.RFC3339,
    }
    s := strings.Trim(string(b), "\"")
    if s == "null" {
        t.Time = time.Time{}
        return
    }
    for _, l := range layout {
        t.Time, err = time.Parse(l, s)
        if err == nil {
            break
        }
    }
    return
}