I need to create a simple application that doesn't work after 30 days.
How can I do that ?
Is there a way to use Trial program after 30 days ?
There is always a way to use a trial program after 30 days.
If by "best" you mean least breakable, then there is really only one way: make it an Internet-based "software as a service" application. If you don't provide access, they're not using the software. If it is a desktop-based application, then some key piece of functionality would still have to be on an online server somewhere.
If the hacker has the entire application on their local machine, it can be reverse engineered and "cracked" in some fashion. The only way to overcome this is to assure that some part of the functionality is never on their machine.
I think a better solution is a feature-limited application -- say you can do basic operations but to take advantage of the best features you have to get a paid license. This does two things -- assuming your app is well-done and interesting to a variety of users. First, you can get a larger number of people to try your app. Second you can get some good will by releasing your "lite" version free.
If you really need to do a time-limited trial, then I would consider having it write an encrypted key to the registry (windows) or a dot-file (linux). This key could encode the expiration date. Using reversible encryption allows you to get this value back out. Using a secret key helps protect against someone generating their own (valid) key. The fully licensed app could use the same key mechanism but encrypting a key value that allows permanent usage.
From my answer to implementing-expiration-dates-in-an-application/1871218#1871218:
Probably the most user friendly to do this is to keep track of the number of days a user has used your software. For example, each time your program starts up you could write a date to an encrypted file (unless the date already exists in the file). Then once there are more than, say, 30 dates in the file, let the user know it is time to buy the full version.
Real products such as Beyond Compare use this scheme - great for potential customers that install your product and don't use it for 30 days only to find that the trial has expired.
Also keep in mind that your scheme does not have to be perfect, just make it strong enough that it will not be convenient for users to break it.
Have the program request a key from your server on first run and check its validity every time the program starts and every 24 hours.
Another option is to have a server-side compiler when you download the application. This can "inject" a license key into the binary. You can do this using a constant in a seperate file. This can then be linked with the other pre-comiled files. This method is slower but harder to crack as the hacker would need to decompile the program.
来源：https://stackoverflow.com/questions/2481320/what-is-the-best-way-to-create-a-program-which-works-only-in-the-predefined-tria