We have a table of timesheet data, with hours spent on a specific date, by a person. 

We also have a table with a persons cost rate between two dates. For example, £25, from 01/04/19, to 31/03/20. This table allows us to track changes in a persons cost rate of the get a salary increase. So the same person may also have a row in addition to the above for: £28, from 01/04/20, to 31/03/21. 

We then have a table for people, which links the two tables above. So the time and cost rate tables both have columns which refer to the people table, where there are many time and cost rate linking to one person. 

The solution we need is a calculation which finds the cost of the time spent. But because we have different cost rates for different periods, but for the same person, it needs to be able to reference the correct cost rate for when the time was spent, then multiply it with that rate to find the cost. 

Any help would be much appreciated 

joe