Set the date permanently to a .txt file and calculate remaining days based on that date using c#

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C#

namespace SimpleLicense
{
    class Program
    {
        static void Main(string[] args)
        {

            string fileName = @"C:\\Temp\\test.txt";

            try
            {
                // Check if file already exists. If yes, delete it. 
                if (File.Exists(fileName))
                {
                    File.Delete(fileName);
                }
                
                // Create a new file 
                Directory.CreateDirectory(Path.GetDirectoryName(fileName));
                
                using (StreamWriter sw = File.CreateText(fileName))
                {
                    
                    sw.WriteLine("Thermo Licensing System file");
                    sw.WriteLine("------------------------------------");
                    sw.WriteLine("Installed Date: {0}", DateTime.Now.ToString());
                    
                    
                    DateTime newDate = DateTime.Now.AddDays(30);
                    DateTime date = DateTime.Now;
                    sw.WriteLine("License Expires After"+" "+newDate);

                    int numberOfDays = newDate.Subtract(date).Days;
                    sw.WriteLine("Number of Days Remaining: " + "  " + numberOfDays.ToString());
                    sw.Close();
                    
                 
                }

                // Write file contents on console. 
                using (StreamReader sr = File.OpenText(fileName))
                {
                    string s = "";
                    while ((s = sr.ReadLine()) != null)
                    {
                        Console.WriteLine(s);
                    }
                         Console.ReadLine();
                }
            }
            catch (Exception Ex)
            {
                Console.WriteLine(Ex.ToString());
            }
        }
    }
}

Contents of .txt File

Thermo Licensing System file
------------------------------------
Installed Date: 20-05-2014 16:01:42
License Expires After 20-06-2014 16:01:42
Number Of Days Remaining

Hi Everyone,

I have written the above code to store date and time information to a .txt file as given above.

I want that when the application is executed on a particular date, the date should be set to that date ie at installed date and should not change and the remaining days be calculated on the basis of Installed date

Can anyone help me to figure this out?

Thanks