https://arstechnica.com/gadgets/2019/03/calc-exe-is-now-open-source-theres-surprising-depth-in-its-ancient-code/

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How to make your system unhackable: delete calc.exe --

calc.exe is now open source; there's surprising depth in its ancient
code

The actual calculation engine is more than 20 years old.

Ars Staff - Mar 7, 2019 5:03 pm UTC

calc.exe is now open source; there's surprising depth in its ancient
code
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Microsoft's embrace and adoption of open source software has
continued with the surprising decision to publish the code for
Windows Calculator and release it on GitHub under the permissive MIT
license.

The repository shows Calculator's surprisingly long history. Although
it is in some regards one of the most modern Windows
applications--it's an early adopter of Fluent Design and has been used
to showcase a number of design elements--core parts of the codebase
date all the way back to 1995.

The actual calculations are performed by this ancient code.
Calculator's mathematics library is built using rational numbers
(that is, numbers that can be expressed as the ratio of two
integers). Where possible, it preserves the exact values of the
numbers it is computing, falling back on Taylor series expansion when
an approximation to an irrational number is required. Poking around
the change history shows that the very earliest iterations of Windows
Calculator, starting in 1989, didn't use the rational arithmetic
library, instead using floating point arithmetic and the much greater
loss of precision this implies.

The project also includes a roadmap for future development. Microsoft
wants to continue to iterate on Calculator's use of Fluent Design,
develop a more complete set of tests, and then add new features as
requested by users. A few bugs and feature requests have already been
filed, and Microsoft has already merged a number of outside
contributions to fix some minor coding errors.

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