Datetime in inner class become '0001-01-01T00:00:00'

[lyk2016]

'date' become '0001-01-01T00:00:00' when use below code

private class TestClass
{
    public TestDate Date1 { get; set; }
}
private class TestDate
{
    public DateTime date;
}

private static Serializer serializer_ = new Serializer(new PropertiesExtractor(), options: GroBufOptions.WriteEmptyObjects);
TestClass cls = new TestClass() { Date1 = new TestDate() { date = DateTime.Now } };
byte[] data = serializer_.Serialize(cls);
var q = serializer_.Deserialize<TestClass>(data);//q.Date1.date become '0001-01-01T00:00:00'

I appreciate your help.
lyk