This depends entirely on what you want to have happen when a trial expires.
You probably want to immediately end the program. You could also give them the option of visiting your website to purchase a license. Maybe something like this:
if (day > 30)
{
if (MessageBox.Show("Trial expired. Visit site to purchase license?",
"Trial Expired!", MessageBoxButtons.YesNo) == DialogResult.Yes)
{
Process.Start("http://www.yourwebsite.com");
}
Environment.Exit(0);
}
If you have something else in mind, or other requirements, update your question with more details.