PK doesn't have to be human readable. And there's value to determining what happened on the Nth of the year over multiple years. Think about how you'd have to deconstruct the key, for instance, if you wanted to compare temperature highs over the last 40 years for the same day.  Yes, you could look at month and day columns, but it's easier if you have a single column, excepting leap years, of course. But that throws anything of this sort off. Therefore, it really depends on what you're trying to do here. 🙂