For a data warehouse to unambiguously report what actually happened in the past it must be able to combine these two timelines.
[1] In bitemporal data models, valid-time and transaction time can be represented two-dimensionally in a Cartesian coordinate system.
In a database table, the transaction time is often represented as an interval allowing the system to "remove" entries by using two table-columns start_tt and end_tt.
Academic researchers and some relational database management systems (RDBMS) have represented until_changed with the largest timestamp supported or the keyword forever.
The term transaction time was coined by Richard T. Snodgrass and his doctoral student Ilsoo Ahn (1986).