Values often need to be tracked as they change over time. Imagine we’re building a payroll system. A foundational requirement would be to store an employee’s pay rate. In all but the most basic of systems, saving this rate as a simple value is insufficient (“Joe’s wage = $20/hour”); rather, a history of the rate over time needs to be maintained (“Joe’s wage = $15/hour as of 10/15/13; Joe’s wage = $20/hour as of 2/1/14”). We do this by giving pay rate a time dimension.

What our system knows about a given moment in time and what really was at that same moment can differ. Suppose Joe’s manager signs paperwork on January 20 giving Joe a pay raise but leaves for vacation before turning in the paperwork. At close of business on January 20, the payroll system knows Joe’s rate to be one amount when, in reality, it’s a different amount. Joe has been given a raise but the system doesn’t yet know about it. Continue reading →