using System;

public class Program

{

    static DateTime RoundUp(DateTimeOffset dt, TimeSpan d)

    {

        return new DateTime(((dt.Ticks + d.Ticks - 1) / d.Ticks) * d.Ticks);

    }

    public static void Main()

    {

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:00"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:01"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:15"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:16"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:29"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:30"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:31"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:44"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:45"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:46"), TimeSpan.FromMinutes(15)).ToString("s"));

        Console.WriteLine();

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:00"), TimeSpan.FromMinutes(30)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:01"), TimeSpan.FromMinutes(30)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:29"), TimeSpan.FromMinutes(30)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:30"), TimeSpan.FromMinutes(30)).ToString("s"));

        Console.WriteLine(RoundUp(DateTimeOffset.Parse("2011-08-11 17:31"), TimeSpan.FromMinutes(30)).ToString("s"));

​

    }

}