Excel 2007 : Excel date formula (tricky)

Theoretically if the invoice date is in the category <=30 (days ago) then that date could be in the current month, last month or even the month before that (e.g. invoice date 31st Jan 2010, today's date 1st March 2010), so I don't know how you want to handle the latter case (if it's 2 months back should it be in 61-90?) but, ignoring that for a moment you could use this sort of approach:

This formula for <=30

=AND($L257<=30,MONTH(TODAY())=MONTH($A257))*$F257

and for 31-60

=AND($L257<=60,OR($L257>30,MONTH(TODAY())<>MONTH($A257)))*$F257

where column A contains the invoice dates

Note: if you don't need column L for other reasons then you could eliminate that column altogether and use =TODAY()-$A257 in the formulas in place of $L257

Edit: simplified suggested formulas