conditional on change of Month

[stew]

Hi All

I have this in Ai Through A120
=INDIRECT("'Invoices to Promoters'!"&ADDRESS(6+42*ROW()-204,(COLUMN())+6))
This picks out the date, inthis form" 1 April 2010 " as a date of the
Sequential Invoices.Various conitations of the same formula pick out values
and local taxes FROM THESE INVOICES over columns B THRU H

I want to ,at the end of every calander month be able to have A SUB TOTAL OF
ALL VALUES IN B THRU H.

COULD I USE AN IF proposistion in conjunction with =INDIRECT("'Invoices to
Promoters'!"&ADDRESS(6+42*ROW()-204,(COLUMN())+6)) ??


Thanks For Looking

Stew