Provenance

A date you can’t trace is a date you have to check yourself. So for every verified year:

1. We cite the official source. An exchange’s own holiday notice, a central bank’s settlement calendar, a government gazette. Each calendar’s page and GET /v1/calendars/{id} list the sources behind it.
2. We keep a copy. Every source page is archived with the time we fetched it and the SHA-256 of what we got, so a date can be traced to the exact page it was checked against, even after the page changes.
3. We watch it. A monitor checks the sources every day. When a page changes, its dates are extracted and compared with ours, and any difference goes to a person for review. Nothing changes the data automatically.
4. We publish the change. Approved fixes go out in a new release, and the changes feed lists exactly which dates changed.

Dates without a verified year come from the holidays library and are labelled library. See Coverage and status.