Interesting. This bug is reproducible in the 'new' Windows 10 calculator store app, but not in the old Windows 7 calc.exe. So looks like a regression, unless they completely re-wrote the whole calculator app (for which there seems to be no reason).
That's actually exactly what happened. The original calc.exe (in all versions up to Windows 8.1) came from Windows 1.0, in plain C, very seldomly updated over the years.
The one in Windows 10 is totally rewritten in C++ using the "Universal Windows Platform" as an API rather than classic Win32.
> tldr; saw a Windows Calculator bug on reddit. Since calc.exe was open-sourced I thought I’d try to find the bug and fix it.
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).