C# Online Compiler | .NET Fiddle

using System;
					
public static class Program
{
	/// <summary>
	/// Rounds datetime up, down or to nearest minutes and all smaller units to zero
	/// </summary>
	/// <param name="dt">static extension method</param>
	/// <param name="rndmin">mins to round to</param>
	/// <param name="directn">Up,Down,Nearest</param>
	/// <returns>rounded datetime with all smaller units than mins rounded off</returns>
	public static DateTime RoundToNearestMinuteProper(this DateTime dt, int rndmin, RoundingDirection directn)
	{
		if (rndmin == 0) //can be > 60 mins
			return dt;
		
		TimeSpan d = TimeSpan.FromMinutes(rndmin); //this can be passed as a parameter, or use any timespan unit FromDays, FromHours, etc.		
		
		long delta = 0;
		Int64 modTicks = dt.Ticks % d.Ticks;
		
		switch (directn)
		{
			case RoundingDirection.Up:
			delta = modTicks != 0 ? d.Ticks - modTicks : 0;
			break;
			case RoundingDirection.Down:
			delta = -modTicks; 
			break;
			case RoundingDirection.Nearest:
			{
				bool roundUp = modTicks > (d.Ticks / 2);
				var offset = roundUp ? d.Ticks : 0;
				delta = offset - modTicks; 
				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
		//10hr 7mins 31secs 
        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
{
    /// <summary>
    /// Rounds datetime up, down or to nearest minutes and all smaller units to zero
    /// </summary>
    /// <param name="dt">static extension method</param>
    /// <param name="rndmin">mins to round to</param>
    /// <param name="directn">Up,Down,Nearest</param>
    /// <returns>rounded datetime with all smaller units than mins rounded off</returns>
    public static DateTime RoundToNearestMinuteProper(this DateTime dt, int rndmin, RoundingDirection directn)
    {
        if (rndmin == 0) //can be > 60 mins
            return dt;
        
        TimeSpan d = TimeSpan.FromMinutes(rndmin); //this can be passed as a parameter, or use any timespan unit FromDays, FromHours, etc.      
        
        long delta = 0;
        Int64 modTicks = dt.Ticks % d.Ticks;
        
        switch (directn)
        {
            case RoundingDirection.Up:

Run-time exception (line 20): Index was outside the bounds of the array.

Stack Trace:

[System.IndexOutOfRangeException: Index was outside the bounds of the array.]
at Program.Main() :line 20