Date and time handling produces some of the cheapest bugs to prevent and the most expensive to find in production. They share three traits that beat ordinary testing: they are rare in wall-clock time (a DST-fallback bug reproduces on one Sunday a year; the 2038 rollover happens exactly once, ever), they need domain research to even write a test case (you have to already know that 2:30 AM did not exist that day in that zone), and they are timezone-dependent, so a bug that is invisible in a UTC CI run still bites a user in Los Angeles. Almost nobody writes a DST test, not out of negligence, but because building the fixture is its own research project. Chronofuzz does that research once and lets you point your function at all of it in a single click.
Paste it into the workbench above and run the battery. Your function is called once per landmine with an ISO string and a target IANA zone, including the US and Sydney spring-forward gaps and the fall-back hour that happens twice. Each result is graded pass, fail, or ambiguous, and failing rows show the exact wrong value next to what a correct implementation returns.
The six categories Chronofuzz ships cover the common ones: DST transitions, leap days (including the 1900 and 2000 century rules), leap seconds, the Unix epoch and the 2038 rollover, ambiguous parsing (bare date strings, slash dates, non-ISO separators), and ISO week-year boundaries where January 1 can belong to the previous year.
No. Everything runs in your browser: JavaScript in a sandboxed Web Worker, Python through Pyodide (CPython compiled to WebAssembly). The sandbox blocks network access, and a shared permalink keeps your paste in the URL fragment, so even a shared link never touches a server.
JavaScript and Python. JavaScript functions are called as
fn(isoInput, timeZone); Python functions are a
def normalize(iso, time_zone):. Toggle between them and each keeps its own
editor buffer.
A fail means there is one correct answer and your function returned a different one, like accepting Feb 29 in a non-leap year. Ambiguous means the input genuinely has no single correct instant, like a fall-back hour or a locale-dependent slash date, and the right behavior is to be explicit about the ambiguity rather than silently pick one.