using System;

public class Program

{

​

    public static void Main()

    {

        DateTime priorExpiration = new DateTime(2017, 12, 31);

        DateTime currentExpiration = new DateTime(2016, 12, 31);

        if (priorExpiration > currentExpiration)

        {

            Console.WriteLine(currentExpiration.MonthDiff(priorExpiration));

        }

    }

}

​

public static class Utils

{

    public static int MonthDiff(this DateTime date1, DateTime date2)

    {

        int difference = (date2.Year - date1.Year) * 12 + date2.Month - date1.Month;

        return difference;

    }

}