DateTimeOffset 18/10 v4, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {    
            // var numEntity
            var numDays = 7;
            var numMonths = 12;            
            
            // This DateTimeOffset will be passed through method from FE
            var parameterStartDay = DateTimeOffset.Now;       
            
            
            // The start of the current WEEK
            DateTimeOffset startOfLastWeek = new DateTimeOffset(new DateTime(parameterStartDay.Year, parameterStartDay.Month, parameterStartDay.Day - 7));            
            List<CampaignCostEntity> dayListCosts = new List<CampaignCostEntity>();            
            for (int i = 0; i <= 7; i++)
            {                
                //float dayTotalCost = Context.Set<Campaign>().Where(x => x.CreatedAt >= startOfLastWeek.AddDays(i) && x.CreatedAt < startOfLastWeek.AddDays(i + 1) && x.CompanyID == shardKey && x.Sent == true && x.Deleted == false).Sum(x => x.Cost);
                //dayListCosts.Add(new CampaignCostEntity() { Name = startOfLastWeek.AddDays(i).DayOfWeek.ToString(), CampaignCost = dayTotalCost });
                dayListCosts.Add(new CampaignCostEntity(){ Name = startOfLastWeek.AddDays(i).DayOfWeek.ToString(), CampaignCost = 1579 });
            }
            foreach (CampaignCostEntity combo in dayListCosts)
                Console.WriteLine(combo.Name + " " + combo.CampaignCost);
                   
            
           
            // The start of the current MONTH
            DateTimeOffset startOfLastMonth = new DateTimeOffset(new DateTime(parameterStartDay.Year, parameterStartDay.Month - 1, parameterStartDay.Day));
            List<CampaignCostEntity> monthListCosts = new List<CampaignCostEntity>();            
            for (int i = 0; i <= 31; i++)
            {                
                //float dayTotalCost = Context.Set<Campaign>().Where(x => x.CreatedAt >= startOfLastWeek.AddDays(i) && x.CreatedAt < startOfLastWeek.AddDays(i + 1) && x.CompanyID == shardKey && x.Sent == true && x.Deleted == false).Sum(x => x.Cost);
                //monthListCosts.Add(new CampaignCostEntity() { Name = startOfLastMonth.AddDays(i).Date.ToString().Substring(0, 5), CampaignCost = dayTotalCost });
                monthListCosts.Add(new CampaignCostEntity(){ Name = startOfLastMonth.AddDays(i).Date.ToString().Substring(0, 5), CampaignCost = 1579 });
            }
            foreach (CampaignCostEntity combo in monthListCosts)
                Console.WriteLine(combo.Name + " " + combo.CampaignCost);            
                       
            
            
            // The start of the current YEAR
            DateTimeOffset startOfLastYear = new DateTimeOffset(new DateTime(parameterStartDay.Year - 1, parameterStartDay.Month, parameterStartDay.Day));
            List<CampaignCostEntity> yearListCosts = new List<CampaignCostEntity>();
            for (int i = 0; i <= 12; i++)
            {                
                //float dayTotalCost = Context.Set<Campaign>().Where(x => x.CreatedAt >= startOfLastWeek && x.CreatedAt < startOfLastWeek && x.CompanyID == shardKey && x.Sent == true && x.Deleted == false).Sum(x => x.Cost);
                //yearListCosts.Add(new CampaignCostEntity() { Name = startOfLastYear.AddMonths(i).Month.ToString(), CampaignCost = dayTotalCost });
                yearListCosts.Add(new CampaignCostEntity(){ Name = startOfLastYear.AddMonths(i).Month.ToString(), CampaignCost = 1579 });
            }
            foreach (CampaignCostEntity combo in yearListCosts)
                Console.WriteLine(combo.Name + " " + combo.CampaignCost);
            
            
            
            Console.WriteLine("Last week starts from: " + startOfLastWeek);
            Console.WriteLine();
            Console.WriteLine("Last month starts from: " + startOfLastMonth);
            Console.WriteLine();
            Console.WriteLine("Last year starts from: " + startOfLastYear);
            Console.WriteLine();
            
            Console.WriteLine( startOfLastWeek.ToString( "dddd" ) );
            Console.WriteLine( startOfLastWeek.AddDays( 1 ));
        }
        
        public class CampaignCostEntity
        {
            public string Name { get; set; }
            public float CampaignCost { get; set; }
        }
    }
}

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Compilation time: 0,2 sec, absolute running time: 0,13 sec, cpu time: 0,11 sec, average memory usage: 13 Mb, average nr of threads: 3