C# Online Compiler | .NET Fiddle

using System;
					
public static class Program
{
	//AI Copilot Improved Code
	public static DateTime RoundToNearestMinuteProper(this DateTime dt, int rndmin, RoundingDirection directn)
	{
		if (rndmin <= 0)
			return dt;

		long ticksPerInterval = TimeSpan.FromMinutes(rndmin).Ticks;
		long modTicks = dt.Ticks % ticksPerInterval;

		long delta = 0;
		switch (directn)
		{
			case RoundingDirection.Up:
				delta = (modTicks == 0) ? 0 : ticksPerInterval - modTicks;
				break;
			case RoundingDirection.Down:
				delta = -modTicks;
				break;
			case RoundingDirection.Nearest:
				delta = (modTicks > ticksPerInterval / 2) ? ticksPerInterval - modTicks : -modTicks;
				break;
			default:
				delta = 0; // Handle unsupported rounding directions
				break;
		}

		return new DateTime(dt.Ticks + delta, dt.Kind);
	}

	 public enum RoundingDirection
	 {
		  Up,
		  Down,
		  Nearest
	 }

	public static void Main()
	{
	  //var dt = new DateTime(2001, 1, 1, 0, 0, 0, 000);
	  //var dt = new DateTime(2018, 08, 03, 10, 7, 30, 000); //8/3/2018 10:00:00 AM Nearest, re 15/2 = 7.5 mins or 7 mins and 30 secs
	  var dt = new DateTime(2018, 08, 03, 10, 7, 31, 000); //8/3/2018 10:15:00 AM Nearest

	  Console.WriteLine(dt.RoundToNearestMinuteProper(15, RoundingDirection.Up)); 
	  Console.WriteLine(dt.RoundToNearestMinuteProper(15, RoundingDirection.Down)); 
	  Console.WriteLine(dt.RoundToNearestMinuteProper(15, RoundingDirection.Nearest)); 
	}
		
}

 
using System;
                    
public static class Program
{
    //AI Copilot Improved Code
    public static DateTime RoundToNearestMinuteProper(this DateTime dt, int rndmin, RoundingDirection directn)
    {
        if (rndmin <= 0)
            return dt;
​
        long ticksPerInterval = TimeSpan.FromMinutes(rndmin).Ticks;
        long modTicks = dt.Ticks % ticksPerInterval;
​
        long delta = 0;
        switch (directn)
        {
            case RoundingDirection.Up:
                delta = (modTicks == 0) ? 0 : ticksPerInterval - modTicks;
                break;
            case RoundingDirection.Down:
                delta = -modTicks;
                break;
            case RoundingDirection.Nearest:
                delta = (modTicks > ticksPerInterval / 2) ? ticksPerInterval - modTicks : -modTicks;

IndexOfMatch: True
ContainsMatch: True
RegExMatch: True