if statement/indexing

I have an employee expense voucher with a column that calculates mileage. I also have a column that lists what the mileage rate is. I'd like for that column to adjust based on the date of the transaction/trip.

Here's an example. Trip between 7/1/08 - 12/31/08 = .585 mileage. Trip between 1/1/09 - 12/31/09 = .405 mileage. If someone turns in a receipt for a trip taken on 11/13/08, I want the form to adjust the mileage rate to .585.

Wasn't sure if this is an indexing kind of thing, or if there's an if statement I could use.