using System;

public class Program

{

    public static void Main()

    {

        DateTime time = new DateTime(2020, 10, 20, 0, 0, 0, 0);

        TimeSpan span = TimeSpan.FromMilliseconds(999);

        //DateTime roundedUp = RoundUp(time, TimeSpan.FromMilliseconds(999));//time.AddMilliseconds(500 - time.Millisecond % 500);

        DateTime roundedUp = time.AddMilliseconds(999* - (time.Ticks) % 999);

        Console.WriteLine($"{span.Ticks}");

        Console.WriteLine($"{time}");

        Console.WriteLine($"{time.Ticks}");

        Console.WriteLine($"{time.Millisecond}");

        Console.WriteLine($"{roundedUp}");

        Console.WriteLine($"{roundedUp.Ticks}");

        Console.WriteLine($"{roundedUp.Millisecond}");

    }

    public static DateTime TimeUp(DateTime time, TimeSpan span)

    {

        long ticks = time.AddTicks(span.Ticks - time.Ticks % span.Ticks).Ticks;

        return new DateTime(ticks, time.Kind);

    }

    public static DateTime RoundUp1(DateTime time, TimeSpan span)

    {

        return new DateTime((time.Ticks + span.Ticks - 1) / span.Ticks * span.Ticks, time.Kind);

    }

    public static DateTime RoundUp(DateTime time, TimeSpan interval)

    {

        if (interval == TimeSpan.Zero)

        {

            // divide by zero exception

            return time;

        }

        return new DateTime(((time.Ticks + interval.Ticks - 1) / interval.Ticks) * interval.Ticks);

    }

}