Find Year Difference using C#

It does not gracefully handle negative timespans, unlike the other algorithm. It also doesn't use its own date arithmetic, instead relying upon the standard library for that.

        static void Main(string[] args)
        {
            / /Check year difference between two dates
            DateTime zeroTime = new DateTime(1, 1, 1);
            DateTime startDate = Convert.ToDateTime(Console.ReadLine());
            DateTime endDate = Convert.ToDateTime(Console.ReadLine());
            if (startDate < endDate)
            {
                DateTime a = new DateTime(startDate.Year, startDate.Month, startDate.Day);
                DateTime b = new DateTime(endDate.Year, endDate.Month, endDate.Day);

                TimeSpan span = b - a;
                // Because we start at year 1 for the Gregorian
                // calendar, we must subtract a year here.
                int years = (zeroTime + span).Year - 1;

                // 1, where my other algorithm resulted in 0.
                Console.WriteLine("Yrs elapsed: " + years);
            }
            Console.ReadLine();
        }