{
    "access_token": "xxx",
    "token_type": "bearer",
    "expires_in": 86400,
    "refresh_token": "yyy",
    "created_at": 1524344276
}

public class OAuth2Token
{
    [JsonProperty("access_token")]
    public string AccessToken  { get; }

    [JsonProperty("token_type")]
    public string TokenType    { get; }

    [JsonProperty("expires_in")]
    public int    ExpiresIn    { get; } //Seconds

    [JsonProperty("refresh_token")]
    public string RefreshToken { get; }

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

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

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteRawValue(((DateTime)value - _epoch).TotalSeconds.ToString());
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.Value == null) { return null; }
        return _epoch.AddSeconds((long)reader.Value);
    }
}