Temporal Data (and some great resources on it)

You see, in a payroll system, you need to know not only what "Dan"'s salary is Today, or what it was on January, you also need to know what you "knew" about Dan's Salary regarding January when you created the payroll for February (assuming today is March, for example). You actually need to keep tabs on time in two dimensions: The actual time, and the history of the actual time. (I always think of it like a comet and it's tail - the comet being the place in time you want to know about and the tail - the value in that time's history you want to know about).
Were you just able to wrap your head around that last paragraph?