How to make my project as 30 days trial

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

namespace TrialTrack
{
    class TrialTimeManager
    {
        private string temp = "";

        public TrialTimeManager(){}

        public void SetNewDate()
        {
            DateTime newDate = DateTime.Now.AddDays(31);
            temp = newDate.ToLongDateString();
            StoreDate(temp);
        }

        public void Expired()
        {
            string d = "";
            using (Microsoft.Win32.RegistryKey key =                Microsoft.Win32.Registry.LocalMachine.OpenSubKey(@"SOFTWARE\MyApp"))
            {
                 d = (String)key.GetValue("Date");
            }
            DateTime now = DateTime.Parse(d);
            int day = (now.Subtract(DateTime.Now)).Days;
            if (day > 30){}
            else if (0 < day && day <= 30){
                 string daysLeft = string.Format("{0} days more to expire", now.Subtract(DateTime.Now).Days);
                 MessageBox.Show(daysLeft);
            }
            else if (day <= 0){
                /* Expiration logic */
            }
        }

        private void StoreDate(string value)
        {
            try
            {
                using (Microsoft.Win32.RegistryKey key =                    Microsoft.Win32.Registry.LocalMachine.CreateSubKey(@"SOFTWARE\MyApp"))
                {
                    key.SetValue("Date", value, Microsoft.Win32.RegistryValueKind.String);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }
}