using System;

public class Program

{

    public static readonly DateTime EPOCH = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

    public static void Main()

    {

        object jsonObject = "253402300800";

​

        object convertedObject = Convert.ChangeType(jsonObject, typeof(long));

        long longTime = (long)convertedObject;

        DateTime expireDateTime = EPOCH.AddMilliseconds(longTime);

        string dateTimeString = expireDateTime.ToString("yyyy-MM-ddTHH:mm:ss:fffZ");

​

        Console.WriteLine(dateTimeString);

    }

}