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7/19/2026
The date string that invented $725 in a synthetic margin report

The date string that invented $725 in a synthetic margin report

Short summary

A detailed bug report from a restaurant margin-review app called LeakLens, where a non-canonical date string (2026-7-9 vs 2026-07-10) caused Python's string sorting to select the wrong 'current day,' producing $725 of false financial impact. The fix enforces canonical YYYY-MM-DD format at the object boundary with regression tests. The app uses deterministic Python for all financial calculations with optional GPT-5.6 narrative, keeping AI away from the math.

  • •Non-canonical date string caused incorrect sorting, producing $725 in false financial scenarios
  • •Fix enforces YYYY-MM-DD format at the DailyMetric object boundary with regression tests
  • •Architecture principle: deterministic code for financial math, AI only for optional narrative explanation

Generated with AI, which can make mistakes.

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