# Time semantics

`valid_at` is business-effective time (which role, kickoff or membership applied at that instant); `known_at` is the knowledge cutoff (what OddsFinder had recorded by that instant, including merges, corrections and backfills). Operations that accept `known_at` echo the effective cutoff in `knowledge`. Answers for a `known_at` at or before the watermark that `GET /v1/coverage` reports for a dataset are stable: nothing recorded at or before that instant is still being accepted, so the same request gives the same answer later. An operation that does not list one of these parameters does not support it and rejects it with the `unsupported-historical-capability` problem rather than silently serving today's state.