Move to SimpleDelegator
Create a gigaseconds_since Date method

Following the Rails approach, let's make this a Date method. Updated the
tests to test both the implementations
Unify implementations

Gigasecond already delegates to Date, so we might as well just use the
Date method implementation
Remove duplicate tests

All these tests do the same thing. Condense down to just one test that
uses a random date, just in case some weird date combos cause problems.

And I like random test inputs because they help show that the input
isn't important, it's the outcome that matters.

Since a single test is so crazy fast, let's run it 2000 times. Why not?