Electricity Planning Engine, part 2: A Reader Comment Found a Real Gap in My Test Suite (and How I Fixed It)

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. Alex's second point, boundaries over averages, is the one I actually had to go check rather than just agree with in the abstract. So I opened tests/Unit/Domain/Contract/PricingStrategyTest.php and looked at every hour used in every peak/off-peak assertion: new DateTimeImmutable ( '2026-07-18 14:00:00' ) // peak new DateTimeImmutable ( '2026-07-18 23:00:00' ) // off-peak new DateTimeImmutable ( '2026-07-18 05:00:00' ) // off-peak 14:00, 23:00, 05:00. Every single one comfortably inside its window. None of them anywhere near the actual transition. The off-peak slot in the config is 22:00 to 06:00 , and the comparison behind that lives in TimeSlot::contains() : // wraparound slot, e.g. 22:00 -> 06:00 return $minuteOfDay >= $this -> startMinuteOfDay || $minuteOfDay < $this -> endMinuteOfDay ; That >= versus < is exactly the kind of one-