lots of exercises in java... from https://github.com/exercism/java

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Gigasecond

Calculate the moment when someone has lived for 10^9 seconds.

A gigasecond is 10^9 (1,000,000,000) seconds.

To run the tests:

$ gradle test

For more detailed info about the Java track see the help page.

Source

Chapter 9 in Chris Pine's online Learn to Program tutorial. http://pine.fm/LearnToProgram/?Chapter=09

It's possible to submit an incomplete solution so you can see how others have completed the exercise.