using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        string s1 = "07-01-2021";

        string s2 = "09-14-2021";

        Console.WriteLine(Convert.ToDateTime(s1).ToString("ddMMMyyyy"));

        Console.WriteLine(Convert.ToDateTime(s2).ToString("ddMMMyyyy"));

        Console.WriteLine("week no "+(GetWeekNo(Convert.ToDateTime(s1),Convert.ToDateTime(s2))));

    }

     public static int GetWeekNo(DateTime d1, DateTime d2, DayOfWeek startOfWeek = DayOfWeek.Monday)

     {

         var diff = d2.Subtract(d1);

​

         var weeks = (int)diff.Days / 7;

​

         // need to check if there's an extra week to count

         var remainingDays = diff.Days % 7;

         var cal = CultureInfo.InvariantCulture.Calendar;

         var d1WeekNo = cal.GetWeekOfYear(d1, CalendarWeekRule.FirstFullWeek, startOfWeek);

         var d1PlusRemainingWeekNo = cal.GetWeekOfYear(d1.AddDays(remainingDays), CalendarWeekRule.FirstFullWeek, startOfWeek);

​

         if (d1WeekNo != d1PlusRemainingWeekNo)

             weeks++;

​

         return weeks+1;

     }

}