Fixing a small calc.exe bug (2019)

2 days ago

A bug in Windows Calculator's date difference feature was discovered, where calculating the difference between July 31st and December 30th incorrectly displayed '5 months, 613566756 weeks, 3 days'.
The bug stems from the non-associative behavior of the AddMonths function, where adding months to a date like July 31st can land on different days depending on the sequence of operations.
The calculation uses unsigned integers, causing negative day differences to wrap around to large numbers when decremented, leading to inflated week counts.
A minimal fix was proposed by adjusting the logic to handle the AddMonths idiosyncrasies, resulting in more accurate but still unintuitive outputs like '4 months, 4 weeks, 1 day'.
Alternative solutions suggested include avoiding unsigned types, using Julian days, leveraging C++11 date libraries, or iterating through calendar units for more human-intuitive results.

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