public class Dog
{
    private const string _birthStringFormat = "yyyy-MM-dd";

    [XmlIgnore]
    public DateTime Birth {get; set;}

    [XmlElement(ElementName="Birth")]
    public string BirthString
    {
        get { return Birth.ToString(_birthStringFormat); }
        set { Birth = DateTime.ParseExact(value, _birthStringFormat, CultureInfo.InvariantCulture); }
    }
}