Make Setup of Windows Application with Serial key allowing only for specific number of pc.

public class ProductKey
    {
        private static DateTime endDate,startDate;
        public ProductKey()
        {

        }
        public ProductKey(DateTime startDate)
        {
            StartDate = startDate;
        }
        public ProductKey(DateTime startDate, DateTime endDate)
        {
            StartDate = startDate;
            EndDate = endDate;
        }
        public ProductKey(DateTime startDate, DateTime endDate,string[] keyParts)
        {
            StartDate = startDate;
            EndDate = endDate;
            KeyParts = keyParts;
        }
        public ProductKey(string[] keyParts)
        {
            KeyParts = keyParts;
        }
        public string[] KeyParts
        {
            get;
            set;
        }
        public DateTime StartDate
        {
            get
            {
                return startDate;
            }
            set
            {
                startDate = value;
            }
        }
        public DateTime EndDate
        {
            get
            {
                return endDate;
            }
            set
            {
                endDate = value;
            }
        }
        public int DaysRemaining
        {
                get
                {
                    if (startDate != null && endDate != null)
                        return endDate.Subtract(startDate).Days;
                    else
                        return 0;
                }
                set
                {
                    endDate = startDate.Add(new TimeSpan(value, 0, 0));
                }
        }
    }