How to enter a formula that uses aspects from 2 different workshee
 

I need to come up with a formula that uses aspects from 2 different
worksheets within the same workbook. The workbook consists of information on
how to figure commission. In one worksheet I have a list of parts that are
originally figured with 3% commission. After one year, parts are then
figured from 2% commission. In a seperate worksheet, I have all the details
for one week's worth of commission. Within this worksheet I want to be able
to create a formula that will look into the parts worksheet and be able to
determine the date of each part and figure which percentage (2% or 3%) should
be used. Can anyone help me??