C# 获取当前年份的周期，周期所在日期范围

最近有一个项目要用到年份周期，用于数据统计图表展示使用，当中用到年份周期,以及年份周期所在的日期范围。当初设想通过已知数据来换算年份周期，经过搜索资料发现通过数据库SQL语句来做，反而更加复杂。现在改变思路通过C#后台代码来算出两段日期范围中年份周期，在依据年份周期所对应的日期范围进行数据库查询进行统计。需要解决以下两个点问题，

第一点：依据日期查找所在年份的第几周；

第二点：依据年份所在的周期计算出周期所在的日期范围。

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication6
{
    class Program
    {
        static void Main(string[] args)
        {
            GregorianCalendar gc = new GregorianCalendar();
            int weekOfYear = gc.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
            Console.WriteLine("当前第{0}周", weekOfYear);
            DateTime startDate, lastDate;
            for (int i = 1; i <= 53; i++)
            {

                GetDaysOfWeeks(DateTime.Now.Year, i, out startDate, out lastDate);
                Console.WriteLine("第{0}周", i);
                Console.WriteLine(startDate);
                Console.WriteLine(lastDate);
            }
            Console.ReadLine();
        }

        public static bool GetDaysOfWeeks(int year, int index, out DateTime first, out DateTime last)
        {
            first = DateTime.MinValue;
            last = DateTime.MinValue;
            if (year < 1700 || year > 9999)
            {
                //"年份超限"
                return false;
            }
            if (index < 1 || index > 53)
            {
                //"周数错误"
                return false;
            }
            DateTime startDay = new DateTime(year, 1, 1);  //该年第一天
            DateTime endDay = new DateTime(year + 1, 1, 1).AddMilliseconds(-1);
            int dayOfWeek = 0;
            if (Convert.ToInt32(startDay.DayOfWeek.ToString("d")) > 0)
                dayOfWeek = Convert.ToInt32(startDay.DayOfWeek.ToString("d"));  //该年第一天为星期几
            if (dayOfWeek == 0) { dayOfWeek = 7; }
            if (index == 1)
            {
                first = startDay.AddDays(7 - dayOfWeek - 6);
                if (dayOfWeek == 6)
                {
                    last = first;
                }
                else
                {
                    last = startDay.AddDays((7 - dayOfWeek));
                }
            }
            else
            {
                first = startDay.AddDays((8 - dayOfWeek) + (index - 2) * 7); //index周的起始日期
                last = first.AddDays(6);
                //if (last > endDay)
                //{
                //    last = endDay;
                //}
            }
            if (first > endDay)  //startDayOfWeeks不在该年范围内
            {
                //"输入周数大于本年最大周数";
                return false;
            }
            return true;
        }
    }
}

执行结果