Date. Let's discuss them.
Time is omnipresent in software development. Pretty much every web application works with it in one way or another. Even the simplest blogging platform stores "when" a post should be published and then displays and sorts them using that information. While you might not run into any issues when developing a blogging platform, any more complex system can easily expose how complicated time actually is. Because time is complicated. And yet, many standard libraries in programming languages only give us a limited arsenal to tackle it.