Average data for last 30 days

I have dates in column J and a corresponding number in column I. Rows of data get added daily. On another sheet I have the average of all the numbers in column I. How can I write a formula to average only the numbers in column I from the last 30 days?

It's more complicated and they are also sorted by other parameters, but I'm keeping the question simple.

Thanks,

Dan

Sweet, I put in the array formula and it seems to work as desired, at least for the situation I've spelled out. Now I have to adapt it for my spreadsheet, which is a little more complicated.

Here is the formula as I entered it to suit the sheet and cell ranges I am using:
=AVERAGE(IF((TODAY()-DWeitsman!J6:J900)<=30,DWeitsman!I6:I900))

I need to adapt the formula to take the same cell ranges from four different worksheets, plus add a condition from column H of each worksheet. Column H contains names, column I contains numerical grades, and I'm averaging grades from each person. I have a formula structure that does this already, but I'm trying to add one that does the last 30 days.

I'd like to attach the spreadsheet so you can see what I'm trying to do, but it's 1.13 MB and the forum won't upload a file that size. Know of a place I can host a spreadsheet?

Thanks,

Dan

Awesome, here's the link to download the file: http://savefile.com/files/1670551

It should open to the "Auditor grades" worksheet, which has average grades given by each person for each name, and their total average grades. Each person has a worksheet that the grades are taken from.

Off to the right side I have entered the array formula you posted, in the row titled Cory Murman. Right now it takes the average grade for ALL the numbers in column I in the sheet DWeitsman, regardless of the name in column H. I want it to take the average grade for only Cory Murman, from all sheets.

By the way, thanks in advance for taking a look at this, I know it's kind of a complicated operation. I bought the Excel Bible and I've been meaning to study up on it so I can do this stuff on my own, but until then I'd like to have a formula so we can start looking at these numbers. I work for a solar energy company and the log items are installations.

Dan

Thanks, I'll try out that formula when I have time to play with it and let you know how it goes. As far as the worksheets, it's not really feasible to put them all together because this is a shared workbook and jobs are entered by all four people throughout the day. If there were only one worksheet, and everybody was adding lines and putting their name into a separate column, there would be editing conflicts when they saved at the end of the day.

Or, I could make a chart of simpler formulas (like what already exists) using the 30-day rule and have a summary exactly like column C. That would probably be easier.