Added calculatable time concept

[teunw]

No description provided.

[jordanbrauer]

I like what you've done! Good job at extending without any dev docs available. 👍 😃

I've got some mixed feelings about the static Year/Month functions – my goal is to keep all complex Unit calculations within the calculate method (also, the calculate method should not need any more args than $value & $to), as well as have as much reusable code as possible. So ... since leap seconds, months, years, etc exist, I think it would be a good idea to make a single method that checks if the current timestamp is a leap period or not.

Perhaps the static functions could be moved onto the abstract TimeUnit class and be merged into a single "::isLeap() : bool" method that utilizes some polymorphism or implements a small strategy pattern to determine what kind of time unit it is checking for a leap period (see Fahrenheit and Celsius using a switch)