A bit of help with a formula will be appreciated.
In column A I have start dates for a series of events and in column B are
the corresponding finish dates. Currently, 72 rows are in use but this list
will continue to grow, 4 - 8 per month. The dates are in the mm/dd/yy
format.
In column C calculated the number of elapsed weeks, =(b2-a2)/7. Formula is
copied down to the last row.
I then calculate a running average for elapsed weeks with =AVERAGE(C:C)/7

What I can not figure out is how to calculate the average elapsed weeks, for
last calendar month ( last month is based upon completion date), and to get
that to advance a month on the first of the next month. Therefore on Feb
1the average elapsed weeks should be calculated for Jan, and on Mar 1, the
calculation changes for elapsed weeks in Feb, etc.

Help will be greatly appreciated.
Paul