The four-timestamp model for forensic traceability of corroboration sources: valid_from (when the fact became true in the world), valid_until (when the fact ceased to be true), ingested_at (when the fact entered the system), and invalidated_at (when the system recognized the fact as no longer current). Bi-temporal provenance makes the engagement record legally defensible, enables retroactive forfeiture detection (sources whose valid_until precedes the current quarter are automatically flagged in entropy rate calculations), and produces a longitudinal dataset quality that compounds with every engagement and is structurally irreproducible retroactively — contributing to the data moat. Applied to corroboration source records in the CC-DATA-01 engagement record schema.