using System;

public class Program

{

    private const long SecondsTo100NsTicks = 10 * 1000;

    private static readonly DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

​

    public static void Main()

    {

        var epochSeconds = 1639994902628;

        var ticks = ConvertEpochSecondsToTicks(epochSeconds);

        var time = new DateTime(ticks).ToLocalTime();

        Console.WriteLine("------ " + DateTime.Now.ToLocalTime());

        Console.WriteLine("------ " + time);

    }

    public static long ConvertEpochSecondsToTicks(long secs)

    {

        var maxSecondsAllowed = (DateTime.MaxValue.Ticks - Epoch.Ticks) / SecondsTo100NsTicks;

        if (secs < 0) secs = 0;

        if (secs > maxSecondsAllowed) secs = maxSecondsAllowed;

        return Epoch.Ticks + secs * SecondsTo100NsTicks;

    }

}