using System.Collections.Generic;

using System.Linq;

using System;

​

public class Program

{

    public static void Main()

    {

        var dt = DateTime.Parse("01.01.2021");

        var input = Enumerable.Range(0, 10)

                        .Select(x => new { Date = dt.AddDays(x), Ammount = 120 })

                        .ToList();

​

        var partitionSize = 5;

        var result = Partition(input, (x) => x.Date, TimeSpan.FromDays(partitionSize))

                        .Select(p=> new { Date = p.Min(x=> x.Date), Ammount= p.Sum(x=>x.Ammount) });

        FiddleHelper.WriteTable(result);

    }

    static IEnumerable<List<T>> Partition<T>(List<T> input, Func<T, DateTime> DateSelector, TimeSpan partitionSize)

    {

        var partitionEnd = DateSelector(input.First()).Add(partitionSize);

        var partition = new List<T>();

​

        foreach (var element in input)

        {

            var dt = DateSelector(element);

            if (dt >= partitionEnd)

            {

                yield return partition;

                partition = new List<T>();

                partitionEnd = dt.Add(partitionSize);

            }

            partition.Add(element);

        }

        yield return partition;

    }

}