Ask HN: What is the most reinvented wheel?

I was reading through an article about html5 inputs and noticed the lack of support for datetime pickers in desktop browsers. Ever website NEEDs to support some form of making date and time picking tolerable, each solution looks and feels different, and nay not always convey the same idea. On mobile however, (datetime-local at least) presents a nice picker that's consistent across sites. Sometimes reinventing the wheel makes sense for educational purposes, but at this point the datetime picker issue is way beyond that. Are there any things in your area of expertise that feel the same way?