using System;

​

public class Program

{

    /// <summary>

    /// Round the datetime

    /// </summary>

    /// <example>Round(dt, TimeSpan.FromMinutes(5)); => round the time to the nearest 5 minutes.</example>

    /// <param name = "dateTime"></param>

    /// <param name = "roundBy">The time use to round the time to</param>

    /// <returns></returns>        

    public static DateTime Round(DateTime dateTime, TimeSpan roundBy)

    {

        long remainderTicks = dateTime.Ticks % roundBy.Ticks;

        if (remainderTicks < roundBy.Ticks / 2)

        {

            // round down

            return dateTime.AddTicks(-remainderTicks);

        }

​

        // round up

        return dateTime.AddTicks(roundBy.Ticks - remainderTicks);

    }

​

    public static void Main()

    {       

        var d = new DateTime(2018, 1, 1, 0, 1, 59, 999);

        var last = d;

        for (int i = 0; i < 100; i++)

        {

            d = d.AddSeconds(29).AddMilliseconds(i * 10);

            var rounded = Round(d, TimeSpan.FromMinutes(1.5));

            if (rounded != last)

            {

                Console.WriteLine("");

            }

​

            Console.WriteLine(String.Format("{0:HH:mm:ss.fff}, Rounded: {1:HH:mm:ss.fff}", d, rounded));

            last = rounded;

        }

    }

}