Time sheet, rates, date dependent

Question

I am trying to figure out the best database design for the following;
I have individuals (stored in Agents table) that enter their billing or time worked each day for different projects. Now we have different rates that we pay based on the person and what
the project is. And we also have to account for individuals getting pay cuts or raises over time. So....
I have the following tables;
  Agents - table of each person
  Projects - table of all projects we work
  Rates - table with AgentID, ProjectID, StartDate, EndDate, Rate
  Billing - table with Date, AgentID, ProjectID
So my current implementation a billing entry must be made by an agent, and I have to create a join to the rates table based on AgentID, ProjectID, and Date between StartDate and EndDate.
This just seems incredibly ugly and all the billing data can be easily changed if a change were made to the rates. I'm assuming most time sheets calculate the total rate and store it in the table so it never changes unless someone specifically goes back to
update the data? I'm just wondering if there is a better method for tracking this sort of data. Thanks

Answer 1

Based on your current database structure 
Agent -   Unique record for each person
Project - Unique record for each project
Rates -   Unique record for agent and project
Billing -   Unique record for agent and project
So basically, you don't need billing table if you are fine with change in billing rate in the middle of any project.  
But if you have to track the Employee cost per project then you might have to keep an entry in Billing table with billing start date, billing end date and active flag (kind of
SCD type 2).
Just a random thought again, you can also have a rate table only and make a new row every time rate is changed or billing is changed. but remember, if rate is changed then you must change the rate of existing employee in that current project (i.e. new row
for all those employee also)