ModelState.IsValid is false on datetime post with EF Database First

I am using Microsoft's Entity Framework 6.1.3. with Database First (from an Azure SQL Server instance). The automatically generated Entity looks like this - the database column is datetime2(0) but really I just want to persist the date but that's another discussion:

public partial class Liquidated
{
    public long ID { get; set; }
    public System.DateTime LiquidationDate { get; set; }
    public string Comment { get; set; }
}

The auto-generated Controller:

    // POST: api/Liquidations
    [ResponseType(typeof(Liquidated))]
    public IHttpActionResult PostLiquidated(Liquidated liquidated)
    {

        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

The unit test using RestSharp:

        var client = new RestClient("http://localhost:64185/api/Liquidations");
        var request = new RestRequest(Method.POST);

        request.AddHeader("cache-control", "no-cache");
        request.AddHeader("authorization", "bearer " + token.access_token);
        request.AddHeader("content-type", "application/x-www-form-urlencoded");
        request.AddHeader("audience", "Any");
        request.AddHeader("accept-language", "en-gb");
        request.AddHeader("accept", "application/json");

        var liquidation = new Liquidated();

        liquidation.LiquidationDate = DateTime.Now;
        liquidation.Comments = "Updated group name 1";

        request.AddObject(liquidation);

        IRestResponse<Liquidated> response = client.Execute<Liquidated>(request);

But when I inspect the ModelState in the Controller it is not valid and shows the following format error for LiquidationDate:

"The value '18/04/2017 14:26:12' is not valid for LiquidationDate."

I've searched many posts, tried to force the formatting (is it a question of date formatting between C# entities and SQL Server?) but I can't see what I'm doing wrong. Any help is much appreciated!