The thirteenth hour of the day starts at 12:00 pm and lasts 60 minutes. Is 1:00 pm part of the thirteenth hour? No.
According to you, the module would answer "yes" if it was easier to do so. Ease has nothing to do it. It's simply behaving correctly.
I was addressing the OP's question about why it might make sense to default time intervals to half-open, rather than closed intervals. Your comment tells me that you define a day as a half-open interval, and you choose to make midnight part of the "following day", not part of the "previous day", a decision I would agree with. In my corner of the world, anyway, we wouldn't get universal agreement on that decision, a proof, I suppose, that if you leave a definition (or interface) ill-defined, confusion will
Where ease comes in is making an interface easy to use correctly, which can be tremendously difficult. From the discussion here, it is quite clear that the interface is not easy to reason about. That would not be fixed by making intervals default to being closed, nor would it be fixed (IMHO) by adding an option to paper over possible confusion by adding "fuzz".