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)); 

    }

}