You could have a formula like this in C1:

=IF(OR(A1="",B1=""),"",B1-A1)

and then copy this down. If either (or both) cell is blank it will
return a blank, and this will not be counted in your AVERAGE formula.

Hope this helps.

Pete

On Jan 26, 12:50*pm, Laurizio
wrote:
Hi,

I'm trying to generate a list of timespans where I eventually want the
average. The timespans (column C) is calculated by formula (B-A). This list
will be constantly updated so I have prepared the formulas for the cells even
though the dates aren't filled in.

An illustration of my spread sheet is:

Row * A * * * * * * * * * B * * * * * * * * * * *C
1 * * * *2009-01-01 * *2009-01-10 * * * *9
2 * * * *2009-01-01 * *2009-01-20 * * * *19
3 * * * *2009-01-05 * *2009-01-10 * * * *5
4 * * * *2009-01-15 * *2009-01-15 * * * *0
5 * * * * * * * * * * * * * * * * * * * * * * * * * * 0 (b5-a5)
6 * * * * * * * * * * * * * * * * * * * * * * * * * * 0 (b6-a6)
...

When I want the average so far it counts with the rows not yet filled in as
well (row 5-6 in the example). My question is; Can I ignore the rows not yet
filled in in any smart way? One option that I have tried is to ignore
everything less than 1 when doing the average formula but then I risk missing
the few ones where the timespan equals 0 (C4 in the example).

Thx for your support!