I have columns set up to record payments (one for the date and one for the
amount). Currently I have columsn enough for 13 payments. So, imagine that
the invoice number is in Col A. Col B has the date the invoice was received.
Col C is for the date of the first payment. Column D is for the amount of
that payment. Col E is for the date of the next payment. Col F is for the
amount of that next payment. And so on.

There will be instances where there won't be anything in Col C-D but there
will be in Col E-F and so on. Some invoices can use all 13 sets of columns
and some might use one and others might use any combination of these 13 sets.

I need to find the average of the dates as compared to when the invoice was
received - basically, how long did it take to pay the invoice. I can't just
average the first column of each 13 sets because if there's no date in the
column, it shouldn't be included.

Does anyone know how to write a formula that will average the number of days
between these dates? I am using Excel 2003. Thanks.