using System;

public class Program

{

    public static void Main()

    {

        var date = DateTime.UtcNow;

        Console.WriteLine(date.AddTicks(-(date.Ticks % TimeSpan.TicksPerSecond)));

        Console.WriteLine(date.Truncate(TimeSpan.FromSeconds(1)));

​

    }

}

​

    public static class DateTimeExtensions

    {

        public static DateTime Truncate(this DateTime dateTime, TimeSpan timeSpan)

        {

            if (timeSpan == TimeSpan.Zero)

            {

                return dateTime;

            }

​

            if (dateTime == DateTime.MinValue || dateTime == DateTime.MaxValue)

            {

                return dateTime; // do not modify "guard" values

            }

​

            return dateTime.AddTicks(-(dateTime.Ticks % timeSpan.Ticks));

        }

    }