C# 30 Days From Todays Date

Question

string dateInString = "01.10.2009";

DateTime startDate = DateTime.Parse(dateInString);
DateTime expiryDate = startDate.AddDays(30);
if (DateTime.Now > expiryDate) {
  //... trial expired
}

DateTime.AddDays does that:

DateTime expires = yourDate.AddDays(30);

DateTime.Now.Add(-30)

Gives you the date 30 days back from now

One I can answer confidently!

DateTime expiryDate = DateTime.Now.AddDays(30);

Or possibly - if you just want the date without a time attached which might be more appropriate:

DateTime expiryDate = DateTime.Today.AddDays(30);

DateTime _expiryDate = DateTime.Now + TimeSpan.FromDays(30);

A possible solution would be on first run, create a file containing the current date and put it in IsolatedStorage. For subsequent runs, check the contents of the file and compare with the current date; if the date difference is greater than 30 days, inform the user and close the application.

A better solution might be to introduce a license file with a counter. Write into the license file the install date of the application (during installation). Then everytime the application is run you can edit the license file and increment the count by 1. Each time the application starts up you just do a quick check to see if the 30 uses of the application has been reached i.e.

if (LicenseFile.Counter == 30)
    // go into expired mode

Also this will solve the issue if the user has put the system clock back as you can do a simple check to say

if (LicenseFile.InstallationDate < SystemDate)
    // go into expired mode (as punishment for trying to trick the app!) 

The problem with your current setup is the user will have to use the application every day for 30 days to get their full 30 day trial.