using System;

using System.Diagnostics;

using System.Threading;

​

public static class Program

{

    public static void Main(string[] args)

    {

        var clock = new StartupClock();

        clock.Start();

​

        while (true)

        {

            var utcNow = clock.UtcNow;

            Console.WriteLine("{0:yyyy/MM/dd HH:mm:ss.fffff}", utcNow);

            Thread.Yield();

        }

    }

}

​

public sealed class StartupClock

{

    long StartupUtcTicks;

    Stopwatch Stopwatch;

​

    public void Start()

    {

        if (Interlocked.CompareExchange(ref StartupUtcTicks, DateTime.UtcNow.Ticks, 0) != 0)

            throw new InvalidOperationException("Start has been executed");

​

        Stopwatch = Stopwatch.StartNew();

    }

​

    public DateTime UtcNow

    {

        get

        {

            var ticks = Interlocked.Read(ref StartupUtcTicks);

            if (ticks == 0)

                throw new InvalidOperationException("Require start called");

​

            return new DateTime(Stopwatch.ElapsedTicks + ticks, DateTimeKind.Utc);

        }

    }

​

    public DateTime LocalNow => UtcNow.ToLocalTime();

}

​