Question on Proximo in Billing terms

[Geert Janssens]

Hi,

I'm trying to setup a billing term that would have my bills due on the 15th of 
the month. I figured out that the "Proximo" option in the billing terms window 
is just for that.

For some reason, I can't get it to work properly.

For example: I have an bill with a bill date of January 2nd 2009. The bill is 
to be due on Januari 15th 2009.

I have a billing term with these settings:
Type: Proximo
Due day: 15
Discount day: 1 (not relevant here)
Discount amount: 0 (not relevant here)
Cutoff day: 0

When I post the bill on January 2nd, the due date becomes February 15th, which 
is one month too late.
I have played with the Cutoff day (set it to +15 or -15), as I understand that 
this would influence the month being picked. But this doesn't change anything. 
In all cases the due date is set as February 15th.

What am I doing wrong ?

Thanks,

Geert