DateTimeOffset 18/10 v1, 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)
        {            
            // 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<DayCampaignCost> daysListCosts = new List<DayCampaignCost>();
            
            for (int i = 0; i <= 7; 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);
                //daysListCost.Add(new DayCampaignCost{ DayName = startOfLastWeek.ToString( "dddd" ), CampaignCost = dayTotalCost });
                daysListCosts.Add(new DayCampaignCost(){ DayName = startOfLastWeek.AddDays(i).ToString( "dddd" ), CampaignCost = 1579 });
            }
            
            foreach (DayCampaignCost combo in daysListCosts)
            {
                Console.WriteLine(combo.DayName + " " + combo.CampaignCost);
            }
            
            
            // The start of the current month
            DateTimeOffset startOfLastMonth = new DateTimeOffset(new DateTime(parameterStartDay.Year, parameterStartDay.Month - 1, parameterStartDay.Day));
           
           // The start of the current year
            DateTimeOffset startOfLastYear = new DateTimeOffset(new DateTime(parameterStartDay.Year - 1, parameterStartDay.Month, parameterStartDay.Day));
            
            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 DayCampaignCost
        {
            public string DayName { get; set; }
            public float CampaignCost { get; set; }
        }
    }
}

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Compilation time: 0,16 sec, absolute running time: 0,09 sec, cpu time: 0,11 sec, average memory usage: 12 Mb, average nr of threads: 3