Make it work with date before 1970

[bubbatls]

The Math.abs was the cause of the weird behavior

[arolson101]

Can you explain why Math.abs was wrong?

[bubbatls]

because the lib needs a signed delta not an unsigned one.

[arolson101]

Sorry, I don't know the code that well.
Would you mind adding a test to your PR?

[bubbatls]

I see the test folder but I dont' understand how to execute them (I use chrome) so I will not add a test without testing it.

[arolson101]

The tests are run via command-line- clone the repo, run npm install, then npm test
If you want you can just reply with an example of it failing I will write the test.

[bubbatls]

I've made a pull request with a test that should work but don't without the current fix

[arolson101]

Thank you!