using System;

using System.Text.RegularExpressions;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        string Data = "{\"aaa\":\"2017-05-12\"}";

        JsonSerializerSettings settings = new JsonSerializerSettings();

        settings.DateParseHandling = DateParseHandling.None;    // I added this line

        settings.Converters.Add(new DateJsonConverter());

        // good

        Xxx x= JsonConvert.DeserializeObject<Xxx>(Data, settings);

        Console.WriteLine(x.Aaa.Year);

        // used to fail - but not anymore!

        Data = "{\"aaa\":\"2017-05-12T08:15:00\"}";

        x = JsonConvert.DeserializeObject<Xxx>(Data, settings);

        Console.WriteLine(x.Aaa.Year);

    }

}

​

public class MyDate

{

    public int Year { get; set; }

    public int Month { get; set; }

    public int Day { get; set; }

}

public class Xxx

{

    public MyDate Aaa { get; set; }

}

​

public class DateJsonConverter : JsonConverter

{

    private readonly Regex _isoDate = new Regex(@"^(\d+)-(\d+)-(\d+).*", RegexOptions.Compiled | RegexOptions.CultureInvariant | RegexOptions.IgnoreCase);

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)

    {

        throw new NotImplementedException();

    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)

    {

        // gives "string" for 2017-04-27 :-)

        // gives DateTime for 2017-04-27T20:00:00.00 :-(

        // I want string only here.

        Console.WriteLine(reader.Value.GetType());

        Match m = _isoDate.Match((string)reader.Value);

        if (!m.Success)

        {

            throw new Exception("Format error for " + (string)reader.Value);

        }

        return new MyDate

        {

            Year = Int32.Parse(m.Groups[1].Value),

            Month = Int32.Parse(m.Groups[2].Value),

            Day = Int32.Parse(m.Groups[3].Value),

        };

    }

    public override bool CanConvert(Type objectType)

    {

        return objectType == typeof(MyDate);

    }

}

​