using System;

public class Program

{

    public static void Main()

    {

        var start = DateTime.Today.AddHours(8).AddMinutes(50);

        var end = DateTime.Today.AddHours(18).AddMinutes(35);

        var inc = 40;

        var startTime = start;

        var endTime = end;

        Console.WriteLine("Start:" + startTime);

        Console.WriteLine("End:" + endTime + "\n");

        while (startTime < endTime.AddMinutes(inc))

        {

            Console.WriteLine("By looping: " + startTime);

            startTime = startTime.AddMinutes(inc);

        }

        startTime = start;

        endTime = end;

        var timeSpan = endTime - startTime;

        var intervals = Math.Ceiling(timeSpan.TotalMinutes / inc);

        Console.WriteLine("\nSpan: " + timeSpan + " (" + timeSpan.TotalMinutes + " min), intervals: " + intervals);

        startTime = startTime.AddMinutes((intervals - 1) * inc);

        Console.WriteLine("By interval calculation: " + startTime);

        startTime = startTime.AddMinutes(inc);

        Console.WriteLine("By interval calculation: " + startTime);

​

    }

}