Using different rates across different time periods

I know potentially answering my own question, but while researching just came across DATESBETWEEN ... could it be as simple as adding DATESBETWEEN (Start, End) on the formula above ... doesn't seem so.  Too easy and trying to think how it would know the dates ... although it understands the Timesheet date from the calendar table.

Or another option, expand the Resource$ table for every resource with every date range so it effectively creates a table of resource$, rates and dates.

I'm thinking of other ways to tackle this one.