If I do understand your need, you have to try the INTNX function (to shift 1 quarter back in the past) with the optional "END" fourth argument to move to the end of the period, that is the last day of the preceding quarter.
[pre]last_quarter_month = INTNX("QUARTER", current_date, -1, "END") ;[/pre]
This gives 31dec2010 for CURRENT_DATE to 01feb2011. If you only want the month, include the whole INTNX function into a MONTH function ; that will produce 12 as a result.
Hope it helps.