App should not work after three days
<p>I am creating an app that has to expire after <strong>three days</strong>. the user should not be able to reinstall the app if it expires. I am storing a file on sdcard at the first run of app, then I want to check the current time of device to that of the creation   time of the file. 
I am not able to calculate differences between two dates . I have written following code.How can I accomplish this task.plz its urgent. thanks in advance.</p>

<pre><code>Date firstFile =  new Date(firstFile.lastModified());
Date currentTime= new Date(currentfile.lastModified());
</code></pre>

<p>I am able to get the values in both variables, but I need to calculate difference in days or minutes</p>
