using System;
 
public class Test
{
	public static void Main()
	{
		DateTime dateTime = DateTime.Now;
		Console.WriteLine($"{dateTime.ToString("yyyy-MM-dd HH:mm:ss.fff")}");
 
		// 밀리초 단위에서 반올림
		Console.WriteLine($"{RoundToNearest(dateTime, TimeSpan.FromSeconds(1.0)).ToString("yyyy-MM-dd HH:mm:ss.fff")}");
 
		// 초 단위에서 반올림
		Console.WriteLine($"{RoundToNearest(dateTime, TimeSpan.FromMinutes(1.0)).ToString("yyyy-MM-dd HH:mm:ss.fff")}");
	}
 
	public static DateTime Truncate(DateTime dateTime, TimeSpan timeSpan)
	{
	    if (timeSpan == TimeSpan.Zero) 
	        return dateTime; 
	    if (dateTime == DateTime.MinValue || 
	        dateTime == DateTime.MaxValue) 
	        return dateTime; 
	    return dateTime.AddTicks(-(dateTime.Ticks % timeSpan.Ticks));
	}
 
	public static DateTime RoundToNearest(DateTime dateTime, TimeSpan timeSpan)
	{
	    long lDelta = dateTime.Ticks % timeSpan.Ticks;
	    bool bIsRoundUp = lDelta > timeSpan.Ticks / 2;
	    long lOffset = bIsRoundUp ? timeSpan.Ticks : 0;
 
	    return new DateTime(dateTime.Ticks + lOffset - lDelta, dateTime.Kind);
	}
}