If you have very looked at the sample for DocumentDB, under the Shared project and in the Until folder, you will see the following class.

namespace DocumentDB.Samples.Shared.Util
{
    using System;
    using System.Collections.Generic;
    using System.Globalization;
    using System.Linq;
    using System.Resources;
    using System.Text;
    using System.Threading.Tasks;
    using Newtonsoft.Json;
    using Newtonsoft.Json.Converters;

    public class UnixDateTimeConverter : DateTimeConverterBase
    {
        private static readonly DateTime UnixStartTime = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            if (value is DateTime)
            {
                long totalSeconds = (long)((DateTime)value - UnixStartTime).TotalSeconds;
                writer.WriteValue(totalSeconds);
            }
            else
            {
                throw new ArgumentException("value");
            }
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            if (reader.TokenType != Newtonsoft.Json.JsonToken.Integer)
            {
                throw new Exception("Invalid token. Expected integer");
            }

            double totalSeconds = 0;

            try
            {
                totalSeconds = Convert.ToDouble(reader.Value, CultureInfo.InvariantCulture);
            }
            catch
            {
                throw new Exception("Invalid double value.");
            }

            return UnixStartTime.AddSeconds(totalSeconds);
        }
    }
}

The primary use of this class is to be used as a converter type for the JsonConverter Attribute as shown in the following code sample.

 [JsonProperty("created_at")]
 [JsonConverter(typeof(UnixDateTimeConverter))]
  public DateTime CreatedAt { get; set; }

The [JsonConverter(Type)] attribute instructs the JsonSerializer to use the  convert  an object to and from JSON

Besides DocumentDB, many social services use a double data type for DateTime values.  Twitter for one, is a good example.  By decorating your models class properties with this attribute,  your DateTime value will be serialized as a UnixDateTime value in JSON format when writing and from JSON  when reading.