Bitemporal
A data model that tracks two time dimensions at once: "valid time" (when a fact was true in the real world) and "transaction time" (when the database learned it). Lets you ask "what did we believe was true on March 1, about events in January?".
Use case: finance, insurance, law — where after-the-fact corrections must be tracked without erasing what the system "believed" before. Example: a salary was registered retroactively valid from January, but only entered in March → bitemporal captures both. SQL:2011 standardized temporal tables (system and application time). Support: MariaDB, SQL Server, Db2, and purpose-built systems (XTDB). Trade-off: powerful auditability but complex queries (each SELECT must specify two time perspectives). Distinct from SCD Type 2 (one time axis) through the double bookkeeping of time.