The bug that differential fuzzing cannot find

I ported a 30-million-download-a-month cron parser from TypeScript to Go and ran 36,000 differential test cases with zero unexplained divergence. Then I found a bug that no amount of fuzzing could ever have caught — because the output wasn't actually a function of the input.

Written for Port Mortem 2026, a porting hackathon run by @partnerships_raptors.

I spent a weekend porting cron-parser, a TypeScript library responsible for roughly 30 million npm downloads a month, to Go.

The result was 2,234 lines of Go replacing 2,823 lines of TypeScript, with zero dependencies. The original relies on Luxon for its date and timezone handling.

But the port itself wasn't the interesting part.

The bug that differential fuzzing cannot find

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