I wrote about the Electricity Planning Engine a little while back, including a timezone bug that made a correct price look "not found" after a database round trip. A few days later, Alex Shev left this comment:
Timezone bugs are brutal in planning engines because the result can look mathematically correct while being operationally wrong. Energy workflows especially need tests around boundaries, not just averages.
That is a genuinely sharp way to put it, and it is not just a comment about the bug I already wrote about. It is a comment about how I test the project in general, and I did not like how well it applied once I went and checked.
The part that stung a little
"Looks mathematically correct while being operationally wrong" is exactly what the original timezone bug was. PriceSeries::priceAt() threw a clean "price not found" error, which is arguably the good version of that failure mode: loud, easy to catch, hard to ship. A quieter version of the same class of mistake, off by one hour instead of missing entirely, would not throw anything. It would just return a plan that looks completely reasonable and is wrong the entire time it runs.
