The Strange Case of the Rogue HP-12c
owl writes:
https://dm319.github.io/pages/2024_09_09_hp12_comma.html
The HP-12c is probably the most iconic financial calculator. Not being in finance myself, and in fact being terribly bad at that kind of thing, I never quite got the purpose of these special-purpose devices. My ignorance came to a halt due to an unfortunate combination of my fixed-rate mortgage period ending and Liz Truss happening, and I was driven to a sudden keen interest in the 'time value of money' (TVM) calculation.
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Earlier this year I came across a fairly benign-looking reddit post describing some difficulty changing the decimal point to a decimal comma on a new Brazilian-bought HP-12c. Most of the replies were along the lines of 'you're holding it wrong', but something caught my attention. They weren't the only one, and not only had someone else had the same experience, I was pointed to numerous Amazon reviews describing similar woes.
To find out for myself, I VPN'd myself over to the Brazilian Amazon and started reading (with the assistance of my phone and google translate) reviews. What I saw was quite consistent - people couldn't change the point to the comma, and the calculator also failed on something called the internal rate of return (IRR) calculation.
I was curious, were these a different version of the HP-12c? Was it a fake? It is generally accepted that the HP-12c (and to some degree the related HP-12c platinum) will return exactly the same results no matter. Why would it otherwise? It was at this point I needed help, and a very kind Brazilian redditor did the work needed to run the aforementioned TVM tests as a forensic tool. What they found was a set of results entirely different to not just the regular HP-12c, but to any other financial calculator we had tested.
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