Dynamic Formulas and Sheet Insertion

I have a spreadsheet containing Sheets for each month of the year, Jan-09 through Apr-09 (this month). Each sheet contains large amounts of data - some of which is based off of a formula ( =H11-Mar-09!H11 ) for example.

It is very awkward to create entirely new formulas each month - is there a way to automatically update the formula upon insertion of a new sheet? We make sure that the sheet containing the most recent month's data is listed first, followed by previous months in order.

A non-VBA solution is preferred, but VBA may be used as a last resort.

I finally found a non-VBA solution! Just in case anyone was wondering:

First, define the following names:

AllSheets
=GET.WORKBOOK(1+0*now())

ThisSheet
=GET.CELL(32+0*now(),indirect("rc",False))

PrevSheet
=INDEX(AllSheets,MATCH(ThisSheet,AllSheets,0)-1+0*now())

NextSheet
=INDEX(AllSheets,MATCH(ThisSheet,AllSheets,0)+1+0*now())

The formula (for my example above) ends up being:

=F11-INDIRECT("'"&NextSheet&"'!"&CELL("address"),F11))

Now, every time I insert a new sheet, the formulas automatically update!

Uh oh - it's not working anymore. I closed the spreadsheet and re-opened it, and now I am getting #name! errors. I am not 100% sure, but my guess is that the GET.WORKBOOK and GET.CELL functions are causing it. Why did these formulas work earlier, but not now?