using System;

public class Program

{

    public static void Main()

    {

        var julianTimestamp = "212569886647992518";

        var timestamp = TimeFromJulianTimestamp(julianTimestamp);

​

        var output = julianTimestamp + " -> " + timestamp.ToString("yyyyMMdd HH:mm:ss.fff");

        Console.WriteLine(output);

    }

​

    public static DateTime TimeFromJulianTimestamp(string julianTimestamp)

    {

        // we know 1 day has 8.64 * (10 to power of 10) microseconds

        // and 2451545.0 in Julian date corresponding to 2000-01-01 12:00:00 (mid day, not midnight) in UTC

        var julianDaysSince = Convert.ToDouble(julianTimestamp) / 8.64e10;

        var diff = julianDaysSince - 2451545.0;

        var referenceDate = new DateTime(2000, 1, 1, 12, 0, 0, 0);

        var dateTime = referenceDate.AddDays(diff);

        return dateTime;

    }

​

​

}