JSON has a weird text format. Here's how to easily convert to it from a C# DateTime:

public string GetJSONFromUserDateTime(DateTime dt)
{
    dt = dt.ToUniversalTime();

    string jsonDateTime = string.Empty;
    if (dt != null)
    {
        JsonSerializerSettings microsoftDateFormatSettings = new JsonSerializerSettings
        {
            DateFormatHandling = DateFormatHandling.MicrosoftDateFormat
        };

        jsonDateTime = JsonConvert.SerializeObject(dt, microsoftDateFormatSettings);
        jsonDateTime = jsonDateTime.Replace("\"\\/Date(", "").Replace(")\\/\"", "");
    }
    return jsonDateTime;
}