Definition of "years"

It will be a shorter book and it will not start four million years ago.

1981, May 5 1718-PDT, Jim McGrath, Earliest Usenet use via Google Groups: fa.sf-lovers, said with a smile at an awards ceremony in the Pennsylvania state Capitol

Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy.

2013 September-October, Katie L. Burke, “In the News”, in American Scientist