Date Arithmetic in Bash

Date and time management libraries in many programming languages are famously bad. Python's datetime module comes to mind as one of the best (worst?) examples, and so does JavaScript's Date class. It feels like these libraries could not have been made worse on purpose, or so I thought until today, when I needed to implement some date calculations in a backup rotation script written in bash.

So, if you wanted to learn how to perform date and time arithmetic in your bash scripts, you've come to the right place. Just don't blame me for the nightmares.