....return the column number where a value is found in an array.

For example:
I have an array, A:AZ where 10/1/2007 is in, say, the fifth column. I want
a formula to return 5.

I have a vlookup that is based on a query output of data from 1/1/2007 to
the current month. One of the summary sheets I am working on only shows the
most recent 13 months. Last month 10/1/2007 was the second column. This
month it would be the first. Next month it would not be there at all and so
on. I want to be able to feed that column position to the column_index_num
in the vlookup.

There are other places in the report where I use all the data returned, so I
cannot limit the data to the most recent 13 months. I currently have two
queries, one for the 13 months and one for all the data. They both contain
the same 13 months of data so I would like to eliminate the redundant piece.

Suggestions? If you have done this and have a better way of doing, feel
free to share!

Thanks!
PJ