Are you able to post a sample spreadsheet? Suggest you post what you currently have (use sample data) and then on another sheet or further down the spreadsheet what you would like to have (anticipated results). I am thinking that if there are dates in the spreadsheet, you might use the effective dates as triggers for an =IF statement that determines which rate to use.

I am guessing something along this line =if(effective date is x, rate1, rate2). If you have multiple effective dates, you may have to nest your IF statment.

HTH
Alan