This works for me, hope it helps.

DateTime currentTime = DateTime.Now;

if (currentTime.Hour >= 5 && currentTime.Hour < 12)
{
    ltWellcome.Text = "Good morning " + strUserName + "!  Welcome to the system";
}
else if (currentTime.Hour >= 12 && currentTime.Hour <= 17)
{
   ltWellcome.Text = "Good afternoon " + strUserName + "!  Welcome to the system";
}
else if (currentTime.Hour >= 18 && currentTime.Hour <= 23)
{
   ltWellcome.Text = "Good evening " + strUserName + "!  Welcome to the system";
}
else
{
   ltWellcome.Text = "Good night " + strUserName + "!  Welcome to the system";
}