Updating formula based on date

Hi

I keep a spreadsheet for statistical purposes of dates certain letters were sent out. I need to be able to look back over the last 2 years and see how many letters were sent out in time. I have a column of dates, Column I, and a column indicating if it was within time (Y) or not (N), Column O.

What I effectively want to do is make the formula look at the list of dates, include only those that are within 2 years of a date of my choosing, then work out a percentage of the number with time (i.e. have a "Y" in the O column). Ideally I would like all this to take place in one box that I can copy and paste and change the date with which to look back over two years.

At the moment I have a cell with my date, a cell with that date - 731 (2 years), a cell for each row with the a formula in column B that effectively says =IF(letterDate<chosenDate,IF(letterDate>twoYearsAgo,withinTime,0),0) and a formula calculating the percentage for the two years =(COUNTIF(B2:B99999,"Y"))/(COUNTIF(B2:B99999,"Y")+COUNTIF(B2:B99999,"N")). If I want to update this for the next quarter I will have to copy and paste the value of that formula and then change the date so that the percentages don't ALL change.

Can anyone help?

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