using System;

using System.Text.Json;

​

​

public class Program

{

    public static void Main(string[] args)

    {

        var rowKey = 2517554849009999999;

        var dt = new DateTime(DateTime.MaxValue.Ticks - rowKey);

        Console.WriteLine($"{rowKey} => {dt:yyyy-MM-dd hh:mm:ss}Z");

        string rowKey2 = string.Format("{0:D19}", DateTime.MaxValue.Ticks - dt.Ticks);

        Console.WriteLine($"{dt:yyyy-MM-dd hh:mm:ss}Z => {rowKey2}");

    }

}

​

​