Get Current Quarter and Last 12 Months, C#

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

using System.Threading;
using System;
namespace Rextester
{
    public static class Program
    {
        public static int GetQuarterName(DateTime myDate)
        {
            return (int)Math.Ceiling(myDate.Month / 3.0);
        }

public static   DateTime GetQuarterStartingDate(DateTime myDate)
        {
            return new DateTime(myDate.Year,(3*GetQuarterName(myDate))-2,1);
        }
         public static   DateTime GetQuarterEndDate(DateTime myDate)
        {
            return new DateTime(myDate.Year,(3*GetQuarterName(myDate)),1).AddMonths(1).AddDays(-1);
        }
        
        public static DateTime Last12MonthsDate(DateTime dateTime)
        {
            DateTime firstDayOfTheMonth = new DateTime(dateTime.Year, dateTime.Month, dateTime.Day);
            return firstDayOfTheMonth.AddMonths(-12).AddDays(-1);
        }
        
        
        public static void Main(string[] args)
        {
           
            var mydate = Convert.ToDateTime("26-08-2020");
            //var mydate = DateTime.Now;
            Console.WriteLine(Math.Ceiling(mydate.Month / 3m));
            Console.WriteLine(GetQuarterStartingDate(mydate));
            Console.WriteLine(GetQuarterEndDate(mydate));
            
             Console.WriteLine(Last12MonthsDate(mydate));
            
        }
        
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,28 sec, absolute running time: 0,13 sec, cpu time: 0,14 sec, average memory usage: 15 Mb, average nr of threads: 3, absolute service time: 0,44 sec