need to automatically change all due date to the fist day of month before domain exp

[rider]

Hi,

 

I only work with domains,

 

I need to automaticaly change all the invoice due date

 

Rule is to get due date to : on month before the domain expiration date, on first day of the month.

 

example if the domain expiration date is 2012/10/28 I need the due date be automatically changed to : 2012/09/01.

 

any idea regarding how to do this by cron ?