using System;

using System.Security.Cryptography;

using System.Text;

​

public class Program

{

    public static void Main()

    {

            DateTime actualExp = new DateTime(2020, 9, 13);

            // Start from hypothetical renew's years

            DateTime zero = new DateTime(1, 1, 1);

​

            // call diff between actual expire and today

            TimeSpan rest = actualExp.Date - DateTime.Now.Date;

            // this days is added on zero years for retrieve the exactly years 

            int yearDurationService = zero.AddDays(rest.Days).Year;

            // So add this year to actual date

            DateTime hypotheticalExpireWithExtension = DateTime.Now.Date.AddYears(yearDurationService);

            // Get difference from new expiration and the actual for give a extension

            TimeSpan diffDays = hypotheticalExpireWithExtension.Date - actualExp.Date;

            Console.WriteLine(diffDays.Days);

    }

}