Non-standard date string silently skewed a restaurant margin report by $725

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A developer building LeakLens, a Python-based restaurant margin-review app, discovered that a non-canonical date string caused the system to analyze the wrong operating day as 'current.' Because Python sorts dates as strings rather than chronological values, '2026-7-9' was ranked after '2026-07-10' alphabetically, making July 9 appear to be the latest day. This reversed comparison generated $725 in false financial scenario impacts from synthetic data, with no visible error or crash to alert the user. The developer chose to reject non-standard date inputs outright rather than silently normalize them, since guessing input formats in a financial tool introduces its own trust risks. Regression tests were added across four boundary cases, all of which failed against the original code, confirming the bug had been accurately identified before any fix was applied.

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