On the semantics of “now” in databases
James Clifford, Curtis Dyreson, Tomás Isakowitz, Christian S. Jensen, Richard Thomas Snodgrass · ACM Transactions on Database Systems · 1997
Although “ now ” is expressed in SQL and CURRENT_TIMESTAMP within queries, this value cannot be stored in the database. How ever, this notion of an ever-increasing current-time value has been reflected in some temporal data models by inclusion of database-resident variables, such as “ now ” “ until-changed, ” “**,” “@,” and “-”. Time variables are very desirable, but their used also leads to a new type of database, consisting of tuples with variables, termed a variable database.