using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

         var input = new[] {

                new Data{Date=DateTime.Parse("01/01/2019 08:00:00"), Field1=1, Field2=1,Field3=1 },

                new Data{Date=DateTime.Parse("01/01/2019 08:15:00"), Field1=1, Field2=1,Field3=1 },

                new Data{Date=DateTime.Parse("01/01/2019 08:30:00"), Field1=1, Field2=1,Field3=1 },

                new Data{Date=DateTime.Parse("01/01/2019 08:31:00"), Field1=1, Field2=1,Field3=1 }

            };

​

            DateTime RoundUp(DateTime dt, TimeSpan d)

                => new DateTime((dt.Ticks + d.Ticks - 1) / d.Ticks * d.Ticks, dt.Kind);

​

            DateTime RoundDown(DateTime dt, TimeSpan d)

                => new DateTime(dt.Ticks - (dt.Ticks % d.Ticks), dt.Kind);

​

            var interval = TimeSpan.FromMinutes(15);

​

            var rangeMin = RoundDown(input.Min(x => x.Date), interval);

            var rangeMax = RoundUp(input.Max(x => x.Date), interval);

​

            var ranges = SplitDateRange(rangeMin, rangeMax, interval).ToList();

​

​

            var temp = from i in input

                       from r in ranges

                       where  r.Includes(i.Date)

                       select

                                new

                                  {

                                      Range = r,

                                      Field1 = i.Field1,

                                      Field2 = i.Field2,

                                      Field3 = i.Field3

                                  };

​

            var result = temp.GroupBy(x => x.Range)

                              .Select(g =>

                                  new

                                  {

                                      Range = g.Key,

                                      SumField1 = g.Sum(x => x.Field1),

                                      SumField2 = g.Sum(x => x.Field2),

                                      SumField3 = g.Sum(x => x.Field3)

                                  })

                               .ToList();

            result.Dump();

    }

    public static IEnumerable<Range<DateTime>> SplitDateRange(DateTime start, DateTime end, TimeSpan ChunkSize)

    {

        DateTime chunkEnd;

        while ((chunkEnd = start.Add(ChunkSize)) < end)

        {

            yield return new Range<DateTime>(start, chunkEnd);

            start = chunkEnd;

        }

        yield return new Range<DateTime>(start, end);

    }

}    

​

    public class Data

    {

        public DateTime Date { get; set; }

        public int Field1 { get; set; }

        public int Field2 { get; set; }

        public int Field3 { get; set; }

    }

​

​

    public struct Range<T> where T : IComparable<T>

    {

        public Range(T start, T end)

        {

            Start = start;

            End = end;

        }

​

        public T Start { get; }

        public T End { get; }

        public bool Includes(T value) => Start.CompareTo(value) <= 0 && End.CompareTo(value) >= 0;

        public bool Includes(Range<T> range) => Start.CompareTo(range.Start) <= 0 && End.CompareTo(range.End) >= 0; 

        public override string ToString(){return "{Start : " +Start+ "\tEnd : "+End+"}";}  

    }