Data contract forcing set datevalue from Web API post

I have a WepApi that is call from a Angular/Json client that serializes to model created in EF code first. I would like to post the product model and set the changedate property inside the post/save function. I get ModelState.IsValid = false, is there any work around I want to keep my ef model property

[Required]  
public DateTime ChangeDate { get; set; }

get this error tray posting

{"$id":"1","Message":"An error has occurred.","ExceptionMessage":"Property 'ChangeDate' on type 'Entity.Product' is invalid. Value-typed properties marked as [Required] must also be marked with [DataMember(IsRequired=true)] to be recognized as required. Consider attributing the declaring type with [DataContract]

CODE model

[DataContract]
public class Product
{
    public Product()
    {
        ChangeDate = DateTime.Now;
    }

    //Serialization
    [DataMember]
    //DataAnnotations
    [DisplayName("ProductId")]
    [Description("ProductId")]
    public int ProductId { get; set; }

    //Serialization
    [DataMember(IsRequired = true)]
    //DataAnnotations
    [DisplayName("ChangeDate")]
    [Description("Date when product last was changed")]
    [Required]
    [DisplayFormat(DataFormatString = "{0:d}")]
    public DateTime ChangeDate { get; set; }

    //Serialization
    [DataMember]
    //DataAnnotations
    [DisplayName("Name")]
    [Description("Name")]
    [Required(ErrorMessage = " ")]
    [StringLength(100)]
    public String Name { get; set; } 


    public HttpResponseMessage Post(Entity.Product product,Boolean directSave = true)
    {
        if (ModelState.IsValid)
        {
            product.ChangeDate = DateTime.Now;
            _productRepository.Add(product);
            if (directSave)
                _context.Save();

            var response = Request.CreateResponse(HttpStatusCode.Created, product);
            response.Headers.Location = GetLocation(product.ProductId);
            return response;
        }

        return Request.CreateResponse(HttpStatusCode.BadRequest);
    }