Android tracking time

After some research, I've decided to extend the hello world app, which was based on the android developer site, by adding some time based functionality.

I think understanding how time works in Java and Android will be a useful tool for me when building applications.  The concept of time is used in many applications to trigger time-based events.  Keeping track of how much time elapses is also important for monitoring and improving the performance of an application.  Finally, understanding how pre-existing Java classes deal with time will be a good introduction, for me, to the Java way of thinking.

In the original app, we start up a main activity with a text box and a button.
After text is entered in the text box, and the button clicked, a second activity is started, with the text message appearing in the second activity.

As a learning experience, here are some things I hope to build into my application:

1. track how long the user has the application in the foreground.
2. track how long the user is in the main activity.
3. Track how long the user is in the second activity (which is started by the first activity).