Group datetimes by duration. Here it will make 3 groups of 15 minutes each

groupbytimespan.cs

using System;
using System.Linq;

namespace GroupbyTimespan {
    class Program {
        static void Main(string[] args) {

            DateTime[] dateTimes = new[]{
                new DateTime(2010, 8, 24, 0, 5, 0),
                new DateTime(2010, 8, 24, 0, 10, 0),
                new DateTime(2010, 8, 24, 0, 15, 0),
                new DateTime(2010, 8, 24, 0, 20, 0),
                new DateTime(2010, 8, 24, 0, 25, 0),
                new DateTime(2010, 8, 24, 0, 30, 0)
            };
            TimeSpan interval = new TimeSpan(0, 15, 0);

            var groupedTimes = from dt in dateTimes
                               group dt by dt.Ticks / interval.Ticks
                               into g
                               select new { Begin = new DateTime(g.Key * interval.Ticks), Values = g.ToList() };

            foreach(var value in groupedTimes) {
                Console.WriteLine(value.Begin);
                Console.WriteLine("\t{0}", String.Join(", ", value.Values));
            }

            Console.ReadLine();

        }
    }
}

output

24.08.2010 00:00:00
        24.08.2010 00:05:00, 24.08.2010 00:10:00
        
24.08.2010 00:15:00
        24.08.2010 00:15:00, 24.08.2010 00:20:00, 24.08.2010 00:25:00
        
24.08.2010 00:30:00
        24.08.2010 00:30:00