Average a range IF within 30 days

Hi,
I have a spreadsheet with dates running down column J, and a number running down column K. I want to average the numbers in column K for all the rows that are less than 30 days old.

I have a formula for determining if a date is less than a month old:
=IF(DATEDIF(J2,TODAY(),"d")<31,TRUE,FALSE)

How can I translate that into averaging a range?

Thanks,

Dan

Edit: I'm using Excel 2003

Thanks. It looks like it should work, but it returns 0 no matter the numbers in the range. Can't figure out why that would be.

Also, does an IF function not have to have a value if false? Because, this one doesn't.

Dan

The spreadsheet is attached. The range I want to average is K6 and down for dates J6 and down.

The dates are formatted as dates; the numer cells are a formula and they work in a simple AVERAGE formula (cell H3). I have placed your formula in N31 to try out (the successful formula will go in H4).

Thanks, that made it work. I didn't know about ctrl+shift+enter, what is the purpose of that?

It might have been working before but I had the cell formatted for no decimals. I put it in H4, formatted for one decimal and it reads 0.5. As data accumulate, the average will become meaningful. There are no data for most days because they don't have a starting datestamp in column C, but the ones below do.

One more question. It seems it doesn't average for cells that read 0. Is there a way to average 0's but not blanks? The 0's are actually date differences between dates entered at the same day, different times, so I thought the calculated value would be some decimal below 1.

Thanks for all your help,

Dan

Wait a second, that was wrong. It looks like it's averaging 0's AND blanks. I want it to not include blanks.

That page is really interesting, thanks for the link.