Additions and tweaks around the date command in particular and ages and dates in general.

• inkarkat/shell-filters for datediff
• automated testing is done with bats - Bash Automated Testing System (https://github.com/bats-core/bats-core)

Download all / some selected extensions (note that some have dependencies, though) and put them somewhere in your PATH, or just add the ./bin subdirectory to PATH.