using System;

public class Program

{

    public static void Main()

    {

​

        DateTime subscriptionStartDate = DateTime.Parse("10/31/20");

        int subscriptionDay = subscriptionStartDate.Day;

​

        DateTime todayDate = DateTime.Now;

        DateTime nextMonthFromToday =  todayDate.AddMonths(1);

        string lastServiceDate ="";

        if(subscriptionDay>todayDate.Day) 

        {

            lastServiceDate =  todayDate.Month.ToString() + "/" +  subscriptionStartDate.Day.ToString() + "/" + todayDate.Year.ToString();

        }

        else

        {

            lastServiceDate = nextMonthFromToday.Month.ToString() + "/" +   subscriptionStartDate.Day.ToString() + "/" + nextMonthFromToday.Year.ToString();

        }

        Console.WriteLine(lastServiceDate);

    }

}