The Windows 10 UWP calculator retains the math engine from before, which actually has been rewritten. The ancient calculator used standard floating-point math, with the fun problems that entails, like 0.1 + 0.1 showing 0.19999999998. The current engine uses rational numbers as long as possible and configurable precision for trigonometry, roots, etc. It also got further upgraded over the years so that the root of a perfect square is always an integer.

As for the bug in question, it could not have happened in the old calculator, as that one didn't have the date math panel.

So for the UWP version they merely redid the UI and added a few things. The actual calculator part is still the same as before (just open-source now).