how do i select which data to use depending on the date?

I need a function that will let me work out how much of an amount to cover
certain date ranges.
I would like to be able to enter any two date ranges eg 1.01.09 and 1.01.12
and use this information to work out the proportion of a salary that is
covered by this range.
the fixed data would be that the salary is $140,000, until 1.06.09 when it
would change to $160,000, and from 1.06.10 salary would be $180,000.
I have been using an IF function and subtracting one date from the other but
it is not working as I get stuck when I put in a start date that occurs after
1.06.09 (the first increase) and also working out how to end the calculation.
I am trying to work out the total salary paid over the given range of dates.

Help would be greatly appreciated.